Calorific value of wood chips. Seminar “Efficient pellet production”
(Fig. 14.1 - Calorific value
fuel capacity)
Pay attention to the calorific value (specific heat of combustion) of various types of fuel, compare the indicators. The calorific value of fuel characterizes the amount of heat released during complete combustion of fuel weighing 1 kg or volume 1 m³ (1 l). Most often, calorific value is measured in J/kg (J/m³; J/l). The higher the specific heat of combustion of the fuel, the lower its consumption. Therefore, calorific value is one of the most significant characteristics of fuel.
The specific heat of combustion of each type of fuel depends on:
- From its flammable components (carbon, hydrogen, volatile combustible sulfur, etc.).
- From its moisture and ash content.
| Table 4 - Specific heat of combustion of various energy carriers, comparative analysis of costs. | |||||||||
| Type of energy carrier | Calorific value | Volumetric density of matter (ρ=m/V) | Unit price standard fuel | Coeff. useful action (efficiency) of the system heating, % | Price per 1 kWh | Implemented systems | |||
| MJ | kWh | ||||||||
| (1MJ=0.278kWh) | |||||||||
| Electricity | - | 1.0 kWh | - | 3.70 rub. per kWh | 98% | 3.78 rub. | Heating, hot water supply (DHW), air conditioning, cooking | ||
| Methane (CH4, temperature boiling point: -161.6 °C) | 39.8 MJ/m³ | 11.1 kWh/m³ | 0.72 kg/m³ | 5.20 rub. per m³ | 94% | 0.50 rub. | |||
| Propane (C3H8, temperature boiling point: -42.1 °C) | 46,34 MJ/kg | 23,63 MJ/l | 12,88 kWh/kg | 6,57 kWh/l | 0.51 kg/l | 18.00 rub. hall | 94% | 2.91 rub. | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, baths, designer lighting |
| Butane C4H10, temperature boiling point: -0.5 °C) | 47,20 MJ/kg | 27,38 MJ/l | 13,12 kWh/kg | 7,61 kWh/l | 0.58 kg/l | 14.00 rub. hall | 94% | 1.96 rub. | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, baths, designer lighting |
| Propane-butane (LPG - liquefied hydrocarbon gas) | 46,8 MJ/kg | 25,3 MJ/l | 13,0 kWh/kg | 7,0 kWh/l | 0.54 kg/l | 16.00 rub. hall | 94% | 2.42 rub. | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, baths, designer lighting |
| Diesel fuel | 42,7 MJ/kg | 11,9 kWh/kg | 0.85 kg/l | 30.00 rub. per kg | 92% | 2.75 rub. | Heating (heating water and generating electricity is very expensive) | ||
| Firewood (birch, humidity - 12%) | 15,0 MJ/kg | 4,2 kWh/kg | 0.47-0.72 kg/dm³ | 3.00 rub. per kg | 90% | 0.80 rub. | Heating (inconvenient to cook food, almost impossible to get hot water) | ||
| Coal | 22,0 MJ/kg | 6,1 kWh/kg | 1200-1500 kg/m³ | 7.70 rub. per kg | 90% | 1.40 rub. | Heating | ||
| MAPP gas (mixture of liquefied petroleum gas - 56% with methyl acetylene-propadiene - 44%) | 89,6 MJ/kg | 24,9 kWh/m³ | 0.1137 kg/dm³ | -R. per m³ | 0% | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, baths, designer lighting | |||
(Fig. 14.2 - Specific heat of combustion)
According to the table “Specific Heat of Combustion of Various Energy Carriers, Comparative Analysis of Costs,” propane-butane (liquefied petroleum gas) is inferior in economic benefits and prospects for use only to natural gas (methane). However, attention should be paid to the tendency towards an inevitable increase in the cost of main gas, which is currently significantly underestimated. Analysts predict an inevitable reorganization of the industry, which will lead to a significant increase in the price of natural gas, perhaps even exceeding the cost of diesel fuel.
Thus, liquefied petroleum gas, the cost of which will remain virtually unchanged, remains extremely promising - the optimal solution for autonomous gasification systems.
Wood is a rather complex material in its chemical composition.
Why are we interested in chemical composition? But combustion (including the burning of wood in a stove) is a chemical reaction of wood materials with oxygen from the surrounding air. The calorific value of firewood depends on the chemical composition of a particular type of wood.
The main chemical binders in wood are lignin and cellulose. They form cells - peculiar containers, inside of which there is moisture and air. Wood also contains resin, proteins, tannins and other chemical ingredients.
The chemical composition of the vast majority of wood species is almost the same. Small fluctuations in the chemical composition of different species determine the differences in the calorific value of different types of wood. Calorific value is measured in kilocalories - that is, the amount of heat obtained by burning one kilogram of wood of a particular species is calculated. There are no fundamental differences between the calorific values of different types of wood. And for everyday purposes it is enough to know the average values.
Differences between rocks in calorific value appear to be minimal. It is worth noting that, based on the table, it may seem that it is more profitable to buy firewood prepared from coniferous wood, because their calorific value is higher. However, on the market, firewood is supplied by volume, not by weight, so there will simply be more of it in one cubic meter of firewood harvested from deciduous wood.
Harmful impurities in wood
During the chemical combustion reaction, wood does not burn completely. After combustion, ash remains - that is, the unburnt part of the wood, and during the combustion process, moisture evaporates from the wood.
Ash has less effect on the combustion quality and calorific value of firewood. Its amount in any wood is the same and is about 1 percent.
But the moisture in wood can cause a lot of problems when burning it. So, immediately after cutting, wood can contain up to 50 percent moisture. Accordingly, when burning such firewood, the lion's share of the energy released with the flame can be spent simply on the evaporation of the wood moisture itself, without doing any useful work.
Moisture present in wood sharply reduces the calorific value of any firewood. Burning wood not only does not perform its function, but also becomes unable to maintain the required temperature during combustion. At the same time, the organic matter in the firewood does not burn completely; when such firewood burns, a large amount of smoke is released, which pollutes both the chimney and the combustion space.
What is wood moisture content and what does it affect?
A physical quantity that describes the relative amount of water contained in wood is called moisture content. Wood moisture content is measured as a percentage.
When measuring, two types of humidity can be taken into account:
- Absolute humidity is the amount of moisture that is currently contained in wood relative to completely dried wood. Such measurements are usually carried out for construction purposes.
- Relative humidity is the amount of moisture that the wood currently contains in relation to its own weight. Such calculations are made for wood used as fuel.
So, if it is written that wood has a relative humidity of 60%, then its absolute humidity will be expressed as 150%.
Analyzing this formula, it can be established that firewood harvested from coniferous trees with a relative humidity of 12 percent will release 3940 kilocalories when burning 1 kilogram, and firewood harvested from deciduous trees with comparable humidity will release 3852 kilocalories.
To understand what a relative humidity of 12 percent is, let us explain that firewood acquires such humidity when it is dried outside for a long time.
Density of wood and its effect on calorific value
To estimate calorific value, you need to use a slightly different characteristic, namely specific calorific value, which is a value derived from density and calorific value.
Information on the specific calorific value of certain wood species was obtained experimentally. The information is given for the same humidity level of 12 percent. Based on the results of the experiment, the following was compiled: table:
Using the data from this table you can easily compare the calorific value of different types of wood.
What kind of firewood can be used in Russia
Traditionally, the most favorite type of firewood for burning in brick kilns in Russia is birch. Although birch is essentially a weed, the seeds of which easily cling to any soil, it is extremely widely used in everyday life. An unpretentious and fast-growing tree has faithfully served our ancestors for many centuries.
Birch firewood has a relatively good calorific value and burns quite slowly and evenly, without overheating the stove. In addition, even the soot obtained from the combustion of birch firewood is used - it includes tar, which is used for both household and medicinal purposes.
In addition to birch, aspen, poplar and linden wood is used as deciduous wood as firewood. Their quality compared to birch, of course, is not very good, but in the absence of others, it is quite possible to use such firewood. In addition, linden firewood, when burned, releases a special aroma, which is considered beneficial.
Aspen firewood produces a high flame. They can be used at the final stage of the fire to burn off soot formed when burning other wood.
Alder also burns fairly smoothly, and after combustion it leaves a small amount of ash and soot. But again, in terms of the sum of all the quality, alder firewood cannot compete with birch firewood. But on the other hand - when used not in a bathhouse, but for cooking - alder firewood is very good. Their even burning helps to cook food efficiently, especially baked goods.
Firewood harvested from fruit trees is quite rare. Such firewood, and especially maple, burn very quickly and the flame reaches a very high temperature during combustion, which can negatively affect the condition of the stove. In addition, you just need to heat air and water in the bath, and not melt metal in it. When using such firewood, it must be mixed with firewood with low calorific value.
Firewood made from softwood is rarely used. Firstly, such wood is very often used for construction purposes, and secondly, the presence of a large amount of resin in coniferous trees pollutes fireboxes and chimneys. It makes sense to heat the stove with pine wood only after long-term drying.
How to prepare firewood
Firewood collection usually begins in late autumn or early winter, before permanent snow cover is established. The felled trunks are left on the plots for initial drying. After some time, usually in winter or early spring, the firewood is removed from the forest. This is due to the fact that during this period no agricultural work is carried out and the frozen ground allows more weight to be loaded on the vehicle.
But this is the traditional order. Now, due to the high level of technological development, firewood can be prepared all year round. Enterprising people can bring you already sawn and chopped firewood any day for a reasonable fee.
How to saw and chop wood
Cut the brought log into pieces suitable for the size of your firebox. Afterwards, the resulting decks are split into logs. Logs with a cross-section of more than 200 centimeters are split with a cleaver, the rest with a regular axe.
The logs are split into logs so that the cross-section of the resulting log is about 80 sq.cm. Such firewood will burn for quite a long time in a sauna stove and produce more heat. Smaller logs are used for kindling.
Chopped logs are stacked in a woodpile. It is intended not just for storing fuel, but also for drying firewood. A good woodpile will be located in an open space, blown by the wind, but under a canopy that protects the wood from precipitation.
The bottom row of woodpile logs is laid on logs - long poles that prevent the firewood from coming into contact with the wet soil.
Drying firewood to an acceptable humidity level takes about a year. In addition, wood in logs dries much faster than in logs. Chopped firewood reaches an acceptable humidity level within three months of summer. When dried for a year, the wood in the woodpile will have a moisture content of 15 percent, which is ideal for combustion.
Calorific value of firewood: video
Environmentally friendly, renewable energy sources
Advantages of using wood chips compared to coal
- When using wood chips, the following issues are resolved:
- disposal of sawmill waste;
- recycling of knots and undergrowth when developing plots in logging areas;
- questions regarding the clearing of agricultural land overgrown with birch and bushes;
- for the utilization of non-commercial wood;
- for clearing forests and shelterbelts;
- reducing the fire hazard of forests and forest belts.
- Boilers running on wood chips are more environmentally friendly because... The boiler automation system monitors the complete combustion of wood chips and the correct ratio of supplied air and wood chips.
- Wood chips are a renewable energy source.
Chip quality
For the smooth functioning of small heating systems, dry, sifted material with certain sizes of individual chips is required. Typically, a material with a particle length of the main fraction from 3.15 to 30 mm and a residual moisture content of less than 30% is used for this.
Larger installations may use coarser materials with increased edge length variations.
An important indicator of combustion quality is the ash content of wood chips. If the ash content is high, flue gas cleaning is required.
Rationing and classification of wood chips
As the main parameters, in accordance with the classification according to the Austrian standard M7133, requirements are established for the size of the chips, for example: G30 - for chips with a cross-section of maximum 3 cm 2, G50 - for chips with a cross-section of maximum 5 cm 2, as well as for moisture content , for example: W35 - for wood chips with a maximum moisture content of 35%.
This standard establishes classes and specifications for the following parameters:
- Humidity
- Ash content
- Fractional composition (size)
- Bulk density
- Nitrogen and chlorine content
- Heat of combustion
Characteristics of woodchips
If the heat of combustion of wood depends only to a small extent on the type of wood, then humidity in this regard is of great importance. In addition, humidity is a determining factor for the shelf life of wood chips.
Wood chips with a moisture content below 30% are classified as “storable”, i.e. in this case we cannot talk about microbial decomposition of wood and the associated losses of mass and energy. The moisture content of freshly cut material ranges from 50% to 60%. Therefore, it is recommended to produce wood chips after preliminary drying.
The following table shows the calorific value depending on humidity. The heat of combustion of freshly cut coniferous trees is approximately 2 kW*h per kg; after drying to a moisture content of 20%, the heat of combustion of wood chips can double (4 kW*h).
Bulk density is the next main characteristic of wood chips (and other solid fuels).
Among other things, it determines the energy density of the fuel and is directly dependent on the volume of space required to store and transport a certain amount of energy.
If the heat of combustion of chips with a moisture content of 20% from oak and beech is 1100 kWh per bulk cubic meter, then the heat of combustion of chips from poplar is significantly lower and amounts to 680 kWh per bulk cubic meter.
For example, to cover the annual demand of 44 MWh of an apartment building, 40 bulk cubic meters of oak and beech chips or 65 bulk cubic meters of poplar chips are required.
Manufacturing and sales
In Germany, chips from coniferous trees are primarily in demand on the market.
In 2007, according to the Federal Bureau of Statistics, the production of chips from coniferous trees amounted to 3.80 million tons, during the same period only 41,000 tons of chips from deciduous trees were produced.
Sales of lower quality croaker and small bushwood products amounted to 1.98 million tonnes. During the same period, 4.04 million tons of chips or blades from coniferous trees and 85,000 tons from deciduous trees were imported. This is a 340% increase in imports over 5 years. 63% of imports came from Austria, the Netherlands and France. Exports of wood chips and flakes in 2007 amounted to 17.94 million tons, which is 66% higher than in 2002.
Price
Wood chip prices have increased over the years, with an 80% increase from July 2004 to July 2009. The retail sales price for dry wood chips in the 4th quarter of 2009 in Germany was 119 Euro per ton (20% moisture content or 25% wood moisture content, delivery 30 m 3, including delivery up to 20 km and VAT). This equates to a liquid fuel equivalent price of 29.71 cents per liter.
Significant differences or fluctuations in price are determined depending on the region, season, quality, humidity and distance from the delivery site. The volume of supply is also an important factor, since powerful CHP plants spend 40% less on fuel than small plants.
FUEL - WOOD CHIPS
Wood chips are shredded wood. The fuel quality is unrivaled, available in sufficient quantities and continuously replenished.
If necessary, only through regular maintenance of your forests can additionally mobilize large quantities each year.
Any unprocessed wood can be processed into chips: round timber, sawmill waste, wood after processing and processing, products from farms with a rapid cutting turnover, trees after thinning and wood residues.
Chips, like pellets:
- Domestic fuel.
- Does not depend on the crisis.
- Neutral to carbon dioxide.
- Not expensive for the price.
Its use reduces dependence on imports, restrains price formation in the country and offers sustainable development opportunities for the regions.
The advantages of wood chips compared to firewood and pieces of wood lie primarily in its flowability, which ensures combustion in fully automatic heating systems.
For the quality of wood chips, such fuel characteristics as moisture, lumpiness, particle size distribution, the proportion of fine fractions, the proportion of bark, bulk density and ash content are important.
With an increase in the proportion of bark during combustion, a larger amount of ash is formed.
Bulk density reflects the weight of a bulk cubic meter and ultimately determines what calorific value the buyer will receive for his money.
In Germany there are no DIN standards for wood chips. Due to long-term use in Germany, the limit values and conditions of the Austrian classification for wood chips in accordance with the Austrian standard M7133 have become established as a trade standard.
In May 2005, a provisional standard (technical specification) entitled “Solid biofuels – Specifications and fuel classes” (DIN CEN/TS 14961) came into force as a classification standard, which defines classes and specifications for the following parameters:
- Humidity
- Ash content
- Grain size distribution
- Bulk density
- Nitrogen and chlorine content
- Heat of combustion
Other data on wood chips:
- Calorific value: OK. 3.3 - 4.3 kW*h/kg or 783 kW*h/m 3 depending on humidity (from freshly cut to 40% humidity).
- Bulk density: OK. 210 - 250 kg/m 3 depending on humidity, 230 kg/m 3 at 20% humidity.
- Ideal size: edge length 30-50 mm.
- Humidity: w (relative humidity) – the mass of water indicated as a percentage in relation to the total mass, the mass of freshly cut wood.
- Dominance: u (absolutely dry wood=absolutely air-dried) – the mass of water indicated as a percentage in relation to the dry mass, the mass of dry matter.
Units:
- 1 Srm = bulk cubic meter, corresponds to 1 m 3 of bulk wood
- 1 rm = folded cubic meter (ster), corresponds to 1 m 3 of wood laid in rows
- 1 fm = 1 cubic meter of solid wood (no gaps)
Conversion factors:
- 1 bulk cubic meter of wood chips = approx. 65-75 l of liquid fuel
- 1 bulk cubic meter of wood chips = bulk density 210-250 kg/m3
- 1 kg wood chips = approx. 3.4 kWh
- 1 folded cubic meter of wood (ster) = approx. 2.5 bulk cubic meters of wood chips
- 1 cubic meter of solid wood = ca. 2.8 bulk cubic meters of wood chips
Primary energy coefficient: for wood chips fP= 0.2
(describes the losses that occur during the receipt, transformation and transportation of the relevant energy carrier)
Calorific value and cost:
Approximate data.
Prices for wood chips may vary by region. (1 ton of wood chips = exactly 3,400 kWh)
The following diagram shows price dynamics since 2007 for wood chips, liquid fuel, gas and pellets per 10 kWh
1 – wood chips, 2 – wood pellets, 3 – liquid fuel, 4 – natural gas.
Humidity
The moisture content of woody biomass is a quantitative characteristic showing the moisture content in the biomass. A distinction is made between absolute and relative humidity of biomass.
Absolute humidity is called the ratio of the mass of moisture to the mass of dry wood:
Where W a is absolute humidity, %; m is the mass of the sample in a wet state, g; m 0 - mass of the same sample, dried to a constant value, g.
Relative or operating humidity The ratio of the mass of moisture to the mass of wet wood is called:
Where W p is relative, or operating, humidity, %
When calculating wood drying processes, absolute humidity is used. In thermal calculations, only relative, or operating, humidity is used. Taking into account this established tradition, in the future we will use only relative humidity.
There are two forms of moisture contained in woody biomass: bound (hygroscopic) and free. Bound moisture is located inside the cell walls and is held by physicochemical bonds; Removing this moisture involves additional energy costs and significantly affects most of the properties of the wood substance.
Free moisture is found in cell cavities and intercellular spaces. Free moisture is retained only by mechanical bonds, is removed much more easily and has less impact on the mechanical properties of wood.
When wood is exposed to air, moisture is exchanged between the air and the wood substance. If the moisture content of the wood substance is very high, this exchange causes the wood to dry out. If its humidity is low, the wood substance is moistened. With a long stay of wood in the air, stable temperature and relative humidity, the moisture content of the wood also becomes stable; this is achieved when the water vapor pressure of the surrounding air becomes equal to the water vapor pressure at the surface of the wood. The amount of stable moisture content in wood kept for a long time at a certain temperature and air humidity is the same for all tree species. Stable humidity is called equilibrium, and it is completely determined by the parameters of the air in which it is located, i.e., its temperature and relative humidity.
Moisture content of stem wood. Depending on the moisture content, stem wood is divided into wet, freshly cut, air-dry, room-dry and absolutely dry.
Wet wood is wood that has been in water for a long time, for example during rafting or sorting in a water basin. The moisture content of wet wood W p exceeds 50%.
Freshly cut wood is wood that has retained the moisture of the growing tree. It depends on the type of wood and varies within the range W p =33...50%.
The average moisture content of freshly cut wood is, %, for spruce 48, for larch 45, for fir 50, for cedar pine 48, for Scots pine 47, for willow 46, for linden 38, for aspen 45, for alder 46, for poplar 48, for warty birch 44, for beech 39, for elm 44, for hornbeam 38, for oak 41, for maple 33.
Air-dry is wood that has been kept in the open air for a long time. While staying in the open air, the wood constantly dries out and its humidity gradually decreases to a stable value. Humidity of air-dried wood W p =13...17%.
Room-dry wood is wood that has been in a heated and ventilated room for a long time. Humidity of room-dry wood W p =7...11%.
Absolutely dry - wood dried at a temperature of t=103±2 °C to constant weight.
In a growing tree, the moisture content of the stem wood is unevenly distributed. It varies both along the radius and along the height of the trunk.
The maximum moisture content of stem wood is limited by the total volume of cell cavities and intercellular spaces. When wood rots, its cells are destroyed, resulting in the formation of additional internal cavities; the structure of rotten wood, as the decay process progresses, becomes loose and porous, and the strength of the wood is sharply reduced.
For these reasons, the moisture content of wood rot is not limited and can reach such high values that its combustion becomes ineffective. The increased porosity of rotten wood makes it very hygroscopic; being in the open air, it quickly becomes moisturized.
Ash content
Ash content refers to the content of mineral substances in the fuel that remain after complete combustion of the entire combustible mass. Ash is an undesirable part of the fuel, as it reduces the content of combustible elements and complicates the operation of combustion devices.
Ash is divided into internal, contained in wood matter, and external, which got into the fuel during the procurement, storage and transportation of biomass. Depending on the type, ash has different fusibility when heated to high temperatures. Low-melting ash is ash that has a temperature of the onset of the liquid-melting state below 1350°C. Medium-melting ash has a temperature of the beginning of the liquid-melting state in the range of 1350-1450 °C. For refractory ash, this temperature is above 1450 °C.
The internal ash of woody biomass is refractory, and the external ash is low-melting.
The ash content of the bark of various species varies from 0.5 to 8% and higher in case of severe contamination during harvesting or storage.
Wood Density
The density of woody matter is the ratio of the mass of the material forming the cell walls to the volume it occupies. The density of wood substance is the same for all types of wood and is equal to 1.53 g/cm3. According to the recommendation of the CMEA commission, all indicators of the physical and mechanical properties of wood are determined at an absolute humidity of 12% and are converted to this humidity.
Density of different types of wood
| Breed | Density kg/m3 | |
| At standard humidity | Absolutely dry | |
| Larch | 660 | 630 |
| Pine | 500 | 470 |
| Cedar | 435 | 410 |
| Fir | 375 | 350 |
| Hornbeam | 800 | 760 |
| White acacia | 800 | 760 |
| Pear | 710 | 670 |
| Oak | 690 | 650 |
| Maple | 690 | 650 |
| Common ash | 680 | 645 |
| Beech | 670 | 640 |
| Elm | 650 | 615 |
| Birch | 630 | 600 |
| Alder | 520 | 490 |
| Aspen | 495 | 470 |
| Linden | 495 | 470 |
| Willow | 455 | 430 |
The bulk density of waste in the form of various shredded wood waste varies widely. For dry chips from 100 kg/m 3, up to 350 kg/m 3 and more for wet chips.
Thermal characteristics of wood
Woody biomass in the form in which it enters the furnaces of boiler units is called working fuel. The composition of woody biomass, i.e. the content of individual elements in it, is characterized by the following equation:
C р +Н р +О р +N р +A р +W р =100%,
where C p, H p, O p, N p are the content of carbon, hydrogen, oxygen and nitrogen in the wood pulp, respectively, %; A p, W p - ash and moisture content in the fuel, respectively.
To characterize fuel in thermal engineering calculations, the concepts of dry mass and combustible mass of fuel are used.
Dry weight In this case, the fuel is biomass dried to an absolutely dry state. Its composition is expressed by the equation
C s +H s +O s +N s +A s =100%.
Combustible mass fuel is biomass from which moisture and ash have been removed. Its composition is determined by the equation
C g + N g + O g + N r = 100%.
The indices of the signs of biomass components mean: p - the content of the component in the working mass, c - the content of the component in the dry mass, g - the content of the component in the combustible mass of fuel.
One of the remarkable features of stem wood is the amazing stability of its elemental composition of combustible mass. That's why The specific heat of combustion of different types of wood is practically the same.
The elemental composition of the combustible mass of stem wood is almost the same for all species. As a rule, the variation in the content of individual components of the combustible mass of stem wood is within the error of technical measurements. Based on this, during thermotechnical calculations, setting up combustion devices that burn stem wood, etc., it is possible to accept the following composition of stem wood for fuel without a large error mass: C g =51%, N g =6.1%, O g =42.3%, N g =0.6%.
Heat of combustion Biomass is the amount of heat released during the combustion of 1 kg of a substance. There are higher and lower calorific values.
Higher calorific value- this is the amount of heat released during the combustion of 1 kg of biomass with the complete condensation of all water vapor formed during combustion, with the release of heat spent on their evaporation (the so-called latent heat of evaporation). The highest calorific value Q in is determined by the formula of D. I. Mendeleev (kJ/kg):
Q in =340С р +1260Н р -109О р.
Net calorific value(NTS) - the amount of heat released during the combustion of 1 kg of biomass, excluding the heat spent on the evaporation of moisture formed during the combustion of this fuel. Its value is determined by the formula (kJ/kg):
Q р =340C р +1030H р -109О р -25W р.
The heat of combustion of stem wood depends only on two quantities: ash content and humidity. The lower heat of combustion of the combustible mass (dry, ash-free!) stem wood is almost constant and equal to 18.9 MJ/kg (4510 kcal/kg).
Types of wood waste
Depending on the production in which wood waste is generated, it can be divided into two types: logging waste and wood processing waste.
Logging waste- These are the separated parts of wood during the logging process. These include needles, leaves, non-lignified shoots, branches, twigs, tips, butts, peaks, trunk cuttings, bark, waste from the production of crushed pulpwood, etc.
In its natural form, logging waste is poorly transportable; when used for energy, it is first crushed into chips.
Wood waste- This is waste generated in woodworking production. These include: slabs, slats, cuttings, short lengths, shavings, sawdust, production waste of industrial chips, wood dust, bark.
Based on the nature of biomass, wood waste can be divided into the following types: waste from crown elements; stem wood waste; bark waste; wood rot.
Depending on the shape and particle size, wood waste is usually divided into the following groups: lump wood waste and soft wood waste.
Lump wood waste- these are cut-offs, peaks, cutouts, slabs, laths, cuts, short lengths. Soft wood waste includes sawdust and shavings.
The most important characteristic of crushed wood is its fractional composition. Fractional composition is the quantitative ratio of particles of certain sizes in the total mass of crushed wood. The crushed wood fraction is the percentage of particles of a certain size in the total mass.
Shredded wood can be divided into the following types according to particle size:
- — wood dust, formed during sanding of wood, plywood and wood boards; the main part of the particles passes through a sieve with a hole of 0.5 mm;
- — sawdust, formed during longitudinal and transverse sawing of wood, they pass through a sieve with holes of 5...6 mm;
- — wood chips obtained by grinding wood and wood waste in chippers; the main part of the chips passes through a sieve with 30 mm holes and remains on a sieve with 5...6 mm holes;
- — large chips, the particle size of which is more than 30 mm.
Let us separately note the features of wood dust. Wood dust generated during sanding of wood, plywood, particle boards and fiberboards cannot be stored either in buffer warehouses of boiler houses or in warehouses for off-season storage of small wood fuels due to its high windage and explosion hazard. When burning wood dust in combustion devices, it is necessary to ensure compliance with all rules for burning pulverized fuel, preventing the occurrence of flashes and explosions inside combustion devices and in the gas paths of steam and hot water boilers.
Wood sanding dust is a mixture of wood particles averaging 250 microns in size with abrasive powder separated from the sanding paper during the sanding process of wood material. The content of abrasive material in wood dust can reach up to 1% by weight.
Features of burning woody biomass
An important feature of woody biomass as a fuel is the absence of sulfur and phosphorus in it. As you know, the main heat loss in any boiler unit is the loss of thermal energy with flue gases. The magnitude of this loss is determined by the temperature of the exhaust gases. When burning fuels containing sulfur, this temperature is maintained at least 200...250 °C in order to avoid sulfuric acid corrosion of the tail heating surfaces. When burning wood waste that does not contain sulfur, this temperature can be lowered to 100...120 °C, which will significantly increase the efficiency of boiler units.
The moisture content of wood fuel can vary within very wide limits. In furniture and woodworking industries, the moisture content of some types of waste is 10...12%; in logging enterprises, the moisture content of the bulk of the waste is 45...55%; the moisture content of bark when debarking waste after rafting or sorting in water basins reaches 80%. Increasing the moisture content of wood fuel reduces the productivity and efficiency of boiler units. The yield of volatiles when burning wood fuel is very high - reaches 85%. This is also one of the features of woody biomass as a fuel and requires a large flame length in which the combustion of combustible components leaving the layer is carried out.
The product of coking woody biomass, charcoal, is highly reactive compared to fossil coals. The high reactivity of charcoal makes it possible to operate combustion devices at low values of the excess air coefficient, which has a positive effect on the efficiency of boiler plants when burning woody biomass in them.
However, along with these positive properties, wood has features that negatively affect the operation of boilers. Such features, in particular, include the ability to absorb moisture, i.e., an increase in humidity in the aquatic environment. With increasing humidity, the lower calorific value quickly drops, fuel consumption increases, combustion becomes more difficult, which requires the adoption of special design solutions in boiler and furnace equipment. At a humidity of 10% and an ash content of 0.7%, the NCV will be 16.85 MJ/kg, and at a humidity of 50% only 8.2 MJ/kg. Thus, the fuel consumption of the boiler at the same power will change by more than 2 times when switching from dry fuel to wet fuel.
A characteristic feature of wood as a fuel is its low internal ash content (does not exceed 1%). At the same time, external mineral inclusions in logging waste sometimes reach 20%. The ash formed during the combustion of pure wood is refractory, and its removal from the combustion zone of the furnace does not present any particular technical difficulty. Mineral inclusions in woody biomass are fusible. When wood with a significant content is burned, sintered slag is formed, the removal of which from the high-temperature zone of the combustion device is difficult and requires special technical solutions to ensure efficient operation of the firebox. The sintered slag formed during the combustion of high-ash wood biomass has a chemical affinity with brick, and at high temperatures in the combustion device it sinteres with the surface of the brickwork of the furnace walls, which makes slag removal difficult.
Heat output usually called the maximum combustion temperature developed during complete combustion of fuel without excess air, i.e., under conditions when all the heat released during combustion is completely spent on heating the resulting combustion products.
The term heat output was proposed at one time by D.I. Mendeleev as a characteristic of the fuel, reflecting its quality from the point of view of its ability to be used for high-temperature processes. The higher the heat output of the fuel, the higher the quality of the thermal energy released during its combustion, the higher the operating efficiency of steam and hot water boilers. Heat output represents the limit to which the actual temperature in the furnace approaches as the combustion process improves.
The heat output of wood fuel depends on its moisture content and ash content. The heat output of absolutely dry wood (2022 °C) is only 5% lower than the heat output of liquid fuel. When the wood moisture content is 70%, the heat output decreases by more than 2 times (939 °C). Therefore, a humidity of 55-60% is the practical limit for using wood for fuel purposes.
The influence of the ash content of wood on its heat performance is much weaker than the influence of humidity on this factor.
The influence of woody biomass moisture content on the efficiency of boiler plants is extremely significant. When burning absolutely dry woody biomass with low ash content, the operating efficiency of boiler units, both in terms of their productivity and efficiency, approaches the operating efficiency of liquid fuel boilers and, in some cases, exceeds the operating efficiency of boiler units using certain types of coal.
An increase in the humidity of woody biomass inevitably causes a decrease in the efficiency of boiler plants. You should know this and constantly develop and carry out measures to prevent atmospheric precipitation, soil water, etc. from getting into wood fuel.
The ash content of woody biomass makes it difficult to burn. The presence of mineral inclusions in woody biomass is due to the use of insufficiently advanced technological processes for wood harvesting and its primary processing. It is necessary to give preference to such technological processes in which the contamination of wood waste with mineral inclusions can be minimized.
The fractional composition of crushed wood should be optimal for this type of combustion device. Deviations in particle size from the optimal, both upward and downward, reduce the efficiency of combustion devices. Chips used to chop wood into fuel chips should not produce large deviations in particle size in the direction of increasing them. However, the presence of a large number of too small particles is also undesirable.
To ensure efficient combustion of wood waste, it is necessary that the design of boiler units meet the characteristics of this type of fuel.
Combustion of fuel is its very rapid chemical destruction and oxidation by atmospheric oxygen, accompanied by heat and light. In this case, carbon forms carbon dioxide, hydrogen-water vapor, oxygen is included in both products, and water evaporates, so that only ash remains from the fuel at the combustion site. Wood fuel is divided into two types: -primary, obtained from logging; - secondary - wood raw materials that were previously used for other purposes (slats, box containers, construction waste, etc.). Advantages of heating with wood: Automatic boiler cleaning Automatic ignition Low emission concentrations Convenient and easy maintenance Firewood is an ecological and renewable energy source Ash can be used as a fertilizer for the garden Elements in wood fuel: Nitrogen Hydrogen Oxygen 0.1% 6.3% 44. 1% Carbon 49.5%
Fuel chips are particles obtained as a result of grinding wood raw materials intended for combustion for energy purposes. Wood fuel chips are produced by processing wood raw materials (stem wood, wood processing waste, wood processing waste and logging residues). Currently, fuel chips from stem wood are most in demand, as they have a number of advantages: Low percentage of bark and other foreign inclusions; Low ash content; High energy value; Standardized particle sizes; Special factories are engaged in the production of fuel chips. Waste generated during the harvesting, processing and processing of wood in the form of knots and branches, tops and sawing waste is crushed into chips using a special chipping machine. To achieve efficiency, the fuel must be homogeneous in fraction, prepared in terms of moisture content. Fuel chips are the basis (raw material) of fuel in power plants for generating electrical energy and heat. Chips are characterized by very diverse indicators depending on humidity, rock, loading method, etc. Chips are counted in cubic meters of dense mass. The average bulk mass of wood chips is assumed to be 0.3 t/m3. Chips of all tree species have a similar chemical composition and contain about 50% carbon. Therefore, the heat of combustion of wood chips of different species in an absolutely dry state per 1 kg is the same: about 18800 kJ/kg (4500 kcal) with deviations of no more than 3-5%.
According to their purpose, wood chips are divided into technological and fuel. Process chips are wood particles intended for the production of cellulose, wood boards, wood chemical and hydrolysis products. Based on their granulometric composition, chips are classified into conditioned, large and fine fractions. Based on the species composition of the raw materials, chips are distinguished from coniferous, deciduous and mixed species. The use of wood chips as fuel makes it possible to profitably dispose of wood waste. The low cost of wood chips makes it a worthy alternative to pellets and briquettes. One of the main characteristics of wood chips is its moisture content. If this figure does not exceed 30%, then such fuel can be stored for a very long time without the danger of biological decomposition and loss of calorific value. The humidity of freshly cut wood is 50-60%, so the raw material must be dried to 30% before grinding.
Types of wood fuel chips: Chips from wood processing waste - chips obtained from unprocessed waste of industrial wood (ribs, butts - the removed butt part of a fallen tree trunk or wood whip, which has processing defects or wood defects, etc.); Stump chips – wood chips obtained from stumps or snags; Chips from logging waste - wood chips obtained from branches and tops (crowns) after harvesting commercial timber; Whole tree chips - chips obtained from the above-ground biomass of a tree (trunk, branches, needles or leaves) Log chips or longwood chips - chips from trees cleared of branches and limbs; Forest chips – wood chips obtained from raw tree wood; Fuel chips – wood chips obtained by grinding for combustion using various methods; Sawmill waste chips are chips obtained from sawmill by-products with or without bark residues;
In general, wood chips are the most stable energy carrier among non-traditional solid fuels, which is due to the following factors: Year-round logging and wood processing, which ensures rhythmic production; Sufficient amount of industrially processed raw materials; Ease of production and use; Stability of the characteristics of wood chips, established by national standards; Low ash content; The high competitiveness of fuel chips compared to other types of alternative solid fuels is emphasized by the market success of wood chips and stable growth rates. Disadvantages of wood chips: Wood fuel chips have a number of disadvantages that are inherent to all wood: Low moisture resistance and ability to absorb moisture, which requires special conditions for storage, transshipment and transportation; Low energy value; High humidity; Low density of fuel chips; Spontaneous combustion; Rapid decay;
One of the promising areas for increasing the energy efficiency of the country’s housing and communal services complex is modernization through the technologies of burning wood fuel produced from low-value wood and logging waste, widely used in Europe. Villages located in the forested regions of the Urals, Siberia and the Far East are forced to purchase fossil fuels (coal, fuel oil, diesel fuel) and transport them hundreds and thousands of kilometers away. At the same time, in the Russian Federation, millions of tons (more than 65) of logging residues, sawmill waste and low-grade wood are practically not used. But such waste, collected within a radius of up to 50 km from a populated area and processed into wood chips, can provide heat even to a regional center, not to mention villages and workers’ settlements. This chips, which is often called fuel or “green” (since twigs, bark, branches with leaves, constituting 20 to 25% of the tree’s biomass, are fed to the shredding machine), is not used in slab and hydrolysis industries. It is economically feasible to harvest it in regions with problematic forest management, where the quality of forests has fallen and forest management becomes unprofitable. One of the most important reasons for the underutilization of the estimated cutting area in such regions is the lack of production facilities for processing low-grade wood (thin wood and dead wood from thinning) and measures for forest reproduction. When switching municipal boiler houses in such areas from coal to wood chips, many problems are solved: new jobs are created, the burden on the local budget is reduced due to a significant reduction in the cost of generated thermal energy.
Considering that coal boiler houses and boiler houses using wood chips have a largely similar design, and the fundamental design of the boilers themselves is identical, many coal boiler houses can be converted to wood chips without replacing the boilers themselves - after minor modernization (in particular, ensuring supply and automation). Co-firing of coal and wood chips can be used, which has long been practiced in many European countries. Of course, not all boiler houses are suitable for implementing such projects. First of all, it is necessary to consider problematic boiler houses, boiler houses in remote areas where the cost of coal is very high, and the raw material base of logging waste in the region is sufficient to produce the required amount of fuel chips. The use of wood chips obtained by grinding low-value wood, logging residues and logging waste will increase not only the energy efficiency of housing and communal services, but also the profitability of logging enterprises, will allow for the effective implementation of forest care and sustainable forestry activities and will improve the environmental situation in forested regions
Heizomat boiler systems Fuel: various forms of biomass - crushed wood (chips), peat, pellets (wood, peat), shavings, sawdust, bark and many others.
Boiler systems Bio. Matic Bio. Control Features: automatic ignition, automatic cleaning of the surfaces of the heat exchanger and burner device from ash, automated removal of ash into attached containers that are easily transported; two-stage regulation of the air supply to the combustion chamber, lambda regulation, temperature control in the screw channels for supplying fuel to the burner, effective thermal insulation. Fuel: wood chips, pellets (diameter 6 mm). Loading: automatic from a separate fuel storage using screw conveyors with mixing systems.
Firematic Bio boiler systems. Control Fuel: wood chips, pellets (diameter 6 mm). Loading: automatic from a separate fuel storage using screw conveyors with mixing systems. Features: automatic ignition, automatic cleaning of the surfaces of the heat exchanger and burner device from ash, automated removal of ash into a front container that is easily transported; two-stage regulation of the air supply to the combustion chamber, lambda regulation, exhaust fan motor with a frequency converter - smooth regulation of the vacuum in the boiler, temperature control in the screw channels for supplying fuel to the burner, effective thermal insulation.
Automation Bio. Control 3000: combustion process control, heating control of the boiler, accumulator tank, control of two heating circuits (pump, three-way valve, flow and return temperature sensors), maintaining a return temperature of 60 0 C (pump, three-way valve, temperature sensor).
And also: Eco wood chips. Smoke Wood chips. In stock! Wholesale and Retail. Price from 10 rub/kg; PF KEDR, IP GOLYSHMANOVSKY INTERFARM FORESTRY (Sawmills and wood processing Tyumen region, A. Ts. Tyumen Yekaterinburg Russian company - exporter "Ural. Mega-Les" is a wholesale supplier of round timber and lumber not only throughout Russia, but also other countries. Deliveries are carried out by rail and sea transport. Med. Plus - Production of lumber LDK A dynamically developing sawmill, equipped with modern high-tech equipment from Laimet, Linck. Main specialization: production of lumber
