A.E. Karapetov, General Director,
LLC "IC KotloProject", St. Petersburg

The article presents an analysis of the design diagrams of boilers and methods of burning biofuel, and also discusses typical mistakes made during the operation of such boilers.

Biofuel combustion stages

Burning is complex process, which consists of sequentially occurring homogeneous and heterogeneous reactions. Basically, combustion occurs in three stages: drying, release and combustion of volatiles, combustion of solid carbon (coke residue). The time required for each of these reactions depends on the characteristics of the fuel, its fractional composition, temperature, and combustion conditions. Experimental combustion of small particles shows a clear separation in time between the phases of combustion of volatiles and coke residue. For larger particles, these phases are superimposed on one another, however, even in fireboxes for burning wood, a fairly clear separation of combustion phases can be observed.

With some combustion methods, such as moving grate, these sequential reactions occur simultaneously in different zones of the boiler combustion chamber, which makes it possible to significantly optimize the combustion process, naturally, provided that the boiler design is correct. In addition, the separation of stages makes it possible to achieve a significant improvement in the environmental performance of the installation as a whole. Fluidized bed combustion, on the contrary, is characterized by the simultaneous occurrence of all three stages of the process in one volume, and under conditions of intense mixing. Thanks to this, the heat released during the combustion of volatiles and coke residue is quickly and efficiently transferred to particles of fresh material and is spent on evaporating moisture and releasing volatiles.

Conditions for efficient combustion

In the English-language literature on biofuel combustion, you can often find the term “Three T"s” - Temperature, Time, Turbulence (“Three T” - temperature, time, turbulence or mixing). These “Three T"s”, three conditions, must be ensured to achieve complete and highly efficient combustion. The main tools for fulfilling the conditions are as follows:

■ correctly selected thermal stress values of the combustion volume and combustion mirror for the combustion method used;

■ configuration of the combustion chamber, ensuring, if necessary, the pressing of hot combustion products to the area to which fresh fuel is supplied, eliminating the presence of stagnant zones, etc.;

■ placement of heat-removing surfaces in combustion and afterburning chambers, taking into account the characteristics, first of all, the humidity of the burned fuel;

■ as uniform a supply of fuel as possible, eliminating single loadings of large portions of fuel;

■ uniform distribution of the fuel layer on the grate (for layer combustion), maintaining the required layer height, ensuring mixing and, if necessary, scuffing the layer;

■ organization of air blast, ensuring a uniform temperature field throughout the volume and cross-section of the combustion chamber;

■ provision operational control behind key parameters (temperature of gases in the combustion zones, afterburning, at the exit from the combustion chamber; O 2 and CO content in the gases leaving the boiler);

■ in order to have an emergency impact on the temperature level in the boiler furnace - organizing flue gas recirculation (as an option - steam injection) into different zones burning.

It is noted that the mixing of flue gases with air should be considered the main factor limiting the quality of biofuel combustion, while ensuring required temperature and residence time in the combustion chamber can be achieved without any problems.

Diagram of air supply to the furnace

The most important parameter that determines the process of biofuel combustion is the excess air α (in English literature the symbol λ is used), which is the ratio of the amount of air supplied to a specific combustion zone to the theoretically necessary one. In biofuel boilers with layered fireboxes and fluidized bed fireboxes, a traditional stepwise air supply to the firebox is used. With this scheme, part of the air (supercharging, primary air) is supplied under the grate, and part is supplied to the area above the grate, possibly in several tiers (afterburning - secondary and tertiary air). Such a scheme is designed to ensure high-quality mixing (turbulence) of the blown air with the products of gasification and incomplete combustion rising from the grate. In this case, it is possible to operate the boiler with low values of the total excess air in the flue gases leaving the boiler, which significantly reduces the amount of heat loss from the flue gases (see Fig. 1).

With a stepped air supply, we can talk about dividing the combustion chamber of the boiler into two zones: a combustion chamber and an afterburning chamber. These zones can simply be located one above the other, as is common in fluidized bed boilers, or they can be structurally separated, in which case the term "pre-furnace" is often used for the combustion chamber. All the fuel and part of the air, the so-called “boost” or primary air, is supplied to the combustion chamber, which is introduced under the fuel layer from below (under the grate). In the combustion chamber, fuel is prepared (evaporation of moisture, release of volatiles) and its partial combustion. When burning wet fuel, a significant amount of thermal energy is required to evaporate the moisture, therefore, as a rule, heat-removing heating surfaces are not placed in the combustion chamber. The “afterburning” secondary air is introduced into the upper part of the combustion chamber or at the entrance to the afterburning chamber. Sometimes, for a more uniform supply, tertiary blast is organized along the flow of gases in the combustion chamber. It is advisable to make afterburning chambers shielded.

Staged combustion, which maintains a reducing atmosphere in the combustion chamber and ensures minimal excess air at the outlet, is an effective primary way to reduce NOx without special (or secondary) measures. Staged combustion allows achieving a reduction in NOx by approximately 50% for fuel with low content nitrogen and approximately 80% for fuel with high content nitrogen). However, to realize this reduction potential, a number of conditions must be met, namely:

■ maintaining the primary air excess coefficient α primary of the order of 0.7 (see Fig. 2);

■ maintaining the temperature in the recovery zone no more than 1150 O C;

■ ensuring the residence time of gases in the reduction zone is at least 0.5 s.

Temperature level in the combustion chamber as a function of the primary air fraction

The main purpose of staging air supply is to avoid temperature peaks in the combustion chamber and, especially, in the combustion chamber (zone). On the one hand, the temperature in the combustion chamber must be high enough to ensure the normal rate of oxidation reactions, but on the other hand, high temperatures cause a number of serious problems:

■ slagging due to melting of fuel ash, which can lead to deterioration of combustion conditions, problems with ash and ash removal equipment, and for fluidized bed furnaces - to disruption of the “boiling” process and boiler shutdown;

■ destruction of the lining, damage to the grate (burnout of the grate);

■ increase in NOx emissions.

For different ways combustion critical temperature values differ. For layer combustion, the value lies in the range of 10501150 O C (in the combustion chamber), and for a fluidized bed it is about 900 O C, which is due to the tendency of the inert material of the layer (sand) to form agglomerates. It is advisable to maintain the temperature in the shielded afterburning chamber at a level not exceeding 1200 ° C.

In the absence of heat-removing heating surfaces in the combustion chamber, the processes that take place in it can, with some degree of convention, be considered adiabatic. In this case, the temperature in the combustion chamber depends on two factors - fuel moisture and excess air. In Fig. 3 these dependencies are presented graphically.

The graph shows that maintaining a subcritical temperature range in the combustion chamber is possible either when working with large excess air, or in a mode below the stoichiometric one. Some negative consequences large excesses of air were discussed above, to them can be added an increased removal of fuel particles from the layer and, accordingly, large values of losses with mechanical underburning in the entrainment, as well as an increase in power consumption due to excess charge air consumption.

Thus, to maintain optimal temperatures in the combustion chamber, the excess air in it should be maintained below stoichiometric, and the lower the moisture content of the supplied fuel, the lower the coefficient of excess primary air α first. Obviously, when burning dry biofuels, as the potential for reducing the temperature due to a decrease in α first is exhausted, it makes sense to consider the issue of placing heat-removing surfaces in the combustion chamber. For fluidized bed furnaces threshold value humidity - about 40%, for layer combustion - 30%.

When burning biofuels with more traditional humidity values Wp, = 45-55% (which is true for wood waste), the following values of α first can be recommended:

■ for combustion in a fluidized bed α first = 0.4-0.55 (the temperature in the bed is 850 ° C);

■ for combustion on moving grates α first = 0.7 (the temperature in the combustion chamber is 1150 °C).

The issue of ensuring the efficiency of secondary air supply

Secondary blast provides the supply of oxidizer to the products of incomplete combustion of fuel leaving the combustion chamber, as well as to small particles of unburned fuel removed from the layer (underburning). The effectiveness of secondary blast can be judged, on the one hand, by the CO content in the exhaust gases and the content of residual carbon in the entrainment and, on the other hand, by the total excess air in the exhaust gases. How less than value all these parameters, the more efficient the secondary air system is. Main factors influencing efficiency:

■ volume of the afterburning chamber, providing required time presence of gases and particles in the zone high temperatures;

■ temperature in the afterburning chamber, ensuring a normal rate of oxidation reactions;

■ “aerodynamics” of the afterburning chamber. This term should be understood as the totality of the geometric configuration of the afterburning chamber, the location of the secondary air nozzles in it, and the range of the jets emerging from them.

Actually, proper organization secondary blast is the organization of such aerodynamics of the afterburning chamber in which:

■ good mixing of combustion products with air is ensured;

■ there are no stagnant zones;

■ a uniform temperature field is ensured;

■ minimum excess air at the outlet is maintained.

It must be borne in mind that the key role in the mixing process is played not by the speed itself, but by the power (or range) of the jet, which depends not only on the speed, but also on the exit diameter of the nozzle. Thus, the same jet power can be obtained by reducing the speed and increasing the diameter, while reducing energy costs to create air pressure in front of the nozzle. Obviously, there must be a certain lower threshold for the speed at which the jet exits the nozzle, after which the system loses efficiency. According to the data for boilers with layered fireboxes given in, the lower limit of the secondary air speed lies in the range of 30-40 m/s.

A separate aspect is the reduction in the speed of air exit from the nozzles and, accordingly, the range of the jets when the boiler is operating at reduced loads. To avoid this, apply the following solutions:

■ use of nozzles with a variable cross-section, which allows for smooth regulation of the outlet cross-sectional area;

■ changing the number of nozzles by switching off using dampers, while discrete regulation of the total outlet cross-sectional area is carried out.

It should be recognized that these solutions are relevant for fairly large boilers, the unit thermal power of which exceeds 20 MW. For boilers of lower power, which are mainly used for burning biofuel, operating at a reduced load with increased excess air is quite acceptable.

Typical errors when operating biofuel boilers

In this subsection I would like to dwell not on the analysis of numerous design schemes of boilers and methods of burning biofuel, but on typical mistakes allowed during the operation of these boilers. In principle, the key mistake is a violation of the correct air balance, namely, working with increased excess air, mainly due to excessive primary blast. The main reason why personnel deliberately increases the air flow rate under the grate is the desire to reduce the temperature in the combustion chamber in order to minimize the risk of destruction of the lining and failure of the grates (in relation to moving grates) or the formation of cakes and agglomerates in the inert material (applied to a fluidized bed). Fear (sometimes irrational) of the gasification regime, in which pops and explosions are possible in the combustion chamber, explains another characteristic operating error - working with an excessive vacuum in the boiler furnace, sometimes up to 100-150 Pa. At the same time, personnel, as a rule, are wary of secondary blast and try, if possible, not to use it.

Both factors together lead to the fact that the oxygen content in the flue gases often reaches, and sometimes exceeds, 10% (α>2). As a result:

■ Boiler efficiency decreases by 4-5% due to increased losses with flue gases compared to normal operation at α = 1.4-1.5;

■ removed from the fuel layer (stationary or boiling) large number particles that do not have time to burn out in the combustion chamber, which leads to an increase in losses with mechanical underburning to a value of q 4 = 3-4%, while values of q 4 = 0.5-1.5% are quite achievable;

■ increased entrainment and underburning in entrainment contribute to sharp growth formation of durable external deposits in the convective heating surfaces of boilers.

These conclusions are based on the author’s experience gained during commissioning, carrying out operational and commissioning tests and participation in the analysis of accidents of biofuel boilers, mainly when using combustion technology in a fluidized bed and on an inclined-pushing grate. For example, gross violations The air mode of operation of the KV-R-11.63-150 boiler, reconstructed for burning oil shale in a fluidized bed (oil shale, of course, is not a biofuel, but is close to it in its reactivity), became the cause of the accident, which developed according to the following algorithm: the gradual clogging of the first convective package along the gases led to a decrease in the cross-section for the passage of gases, the velocities in the remaining section increased many times, due to erosive wear, fistulas formed in several pipes and, as a result, cementation of deposits occurred over almost the entire surface of the package.

As a result of the unsatisfactory performance of boilers of the KVD-1.2M type installed in the boiler room in the village. Lyaskelya (Republic of Karelia) and burning wood waste with a humidity of 50-55%, there was, in addition to extremely low efficiency (less than 70%) and a serious lack of power, a large carryover of unburnt particles from the boiler, which were carried out of the chimney and deposited in the area adjacent to the boiler . As a result of the survey (carried out in 2007), the reasons for both the design plan were identified - insufficient volume of the combustion chamber, poor location of the secondary air nozzles, insufficient heating surface, inoperative ash collection unit, and the operating plan - working with excess air in the flue gases α =2.1-2.6, vacuum behind the boiler 210-240 Pa.

Another example: during the regime adjustment of the KVm-3.0 D hot water boiler with a thermal power of 3 MW in 2014, it was possible to achieve a noticeable increase in the boiler efficiency (by 5-7%) almost only by optimizing the air regime. The boiler burned fine-fraction waste of high dryness (W t r<15%) на наклонно-переталкивающей решетке. В данном случае конструкция котла была лишена недостатков, за исключением не совсем продуманной системы подвода вторичного воздуха. Перед началом наладочных испытаний котел эксплуатировался с сильно завышенным первичным дутьем (т.е. при высоких значениях α перв), вследствие чего топливо выгорало (и частично выносилось), не достигая последних рядов колосников решетки, т.е. почти вся зола покидала котел с уносом, разрежение поддерживалось в диапазоне 80-100 Па, температура газов в камере сжигания (неэкранированной) не превышала 750 О С, избыток воздуха в уходящих газах достигал α=2. Путем перенастройки воздушного режима в сторону значительного сокращения первичного дутья и снижения разрежения в топке до 40-50 Па удалось достичь:

■ uniform distribution and combustion of fuel along the entire length of the grate;

■ reducing the share of fly ash from almost 100% to 55%, while the content of combustibles in the slag did not exceed 7.2%;

■ gas temperature in the combustion chamber is about 880 ° C;

■ coefficient of excess air in flue gases α=1.36.

The characteristics of the boiler after operational adjustment are given in the table.

Parameter	Magnitude
Thermal power, MW	3
Excess air coefficient in flue gases	1,36
Flue gas temperature, °C	198
Heat loss with flue gases, %	9,26
CO content in flue gases (referenced to 0 °C), mg/nm 3	581
Heat loss from chemical incomplete combustion, %	0,2
Residual carbon content in slag, %	7,2
Proportion of fuel ash in slag, %	45,6
Heat loss from mechanical incomplete combustion in slag, %	0,08
Residual carbon content in entrainment, %	32,7
Proportion of fuel ash in entrainment, %	54,4
Heat loss from mechanical incomplete combustion in entrainment, %	0,58
Total heat loss from mechanical incomplete combustion, %	0,66
Heat loss to the environment, %	2,14
Heat loss from slag (at t mn =600 °C), %	0,03
Gross boiler efficiency, %	87,7

Quite large values of the content of combustibles in the entrainment and CO in the flue gases are explained by the already mentioned unsatisfactory operation of the secondary air system, which does not provide effective mixing (“turbulence”) of after-combustion air with combustion products.

Conclusion

How to convey to operating personnel information about the correct methods of controlling biofuel boilers, allowing to reveal all the capabilities of the equipment? How to replace the established concepts of safe and economical operation, which came from the experience of operating old coal boilers, in which sometimes there was no secondary blast at all? It is known that not all boiler houses of small capacity have professionally executed regime maps, and where they exist, compliance with the regimes is not always monitored.

It seems that the most effective way to solve this problem is to minimize the impact of the human factor on the process of controlling the operation of the boiler, i.e. deep degree of automation of the combustion process. This approach is successfully implemented on medium-power boilers; an example is fluidized bed steam boilers for burning wood waste, in the development of which the author had the opportunity to participate. The air mode of these boilers is maintained as follows: the primary air flow rate is strictly linked to the fuel supply, and the secondary air fan is controlled by a regulator based on the oxygen content in the flue gases. This scheme allows you to maintain operation with minimal excess air, the real values of the oxygen content in the gases behind the boiler are O 2 = 3-5% (Fig. 4).

Rice. 4. Display of the control panel of the KE-25-24-350 boiler, reconstructed for burning wood waste in a fluidized bed. Object - Vileika mini-CHP, Vileika, Republic of Belarus. The regulator maintains the oxygen content in the gases behind the boiler (before the steel economizer) O 2 =3%.

It is clear that equipping low-power plants with a developed automation system will significantly affect their cost, but one must understand that this increase in cost will be compensated by higher efficiency. In any case, along this path - complete automation of biofuel boilers, even of low power -

leading foreign equipment suppliers are coming. In addition to oxygen sensors, the installations are equipped with sensors for measuring CO in flue gases, which makes it possible to reduce excess air to the level of gas boilers.

Literature

1. Nussbaumer, Thomas. Combustion and Co-combustion of Biomass: Fundamentals, Technologies, and Primary Measures for Emission Reduction. Energy & Fuels. T. 17. 2003.

2. Sjaak Van Loo, Jaap Koppejan. The Handbook of Biomass Combustion and Co-firing. London: EARTHSCAN, 2008.

3. V.N. Shemyakin, A.E. Karapetov, S.V. Krylov. Experience in practical implementation of fluidized bed technology in industrial and municipal energy. Proceedings of CKTI. OJSC NPO TsKTI, 2009, 298 p.

4. Nitskevich E.A. Design of boiler units. M.: State Energy Publishing House, 1951.

5. Alexandrov V.G. Steam boilers of low and medium power. L.: Energy, 1972.

6. Ivanov Yu.V. Efficient combustion of above-layer combustible gases in furnaces. Tallinn: Estonian State Publishing House, 1959.

The systematization of biofuel combustion technologies is quite complex and confusing. This is due to the high rate of development of biofuel consumption by humanity. A huge amount of biofuel raw materials and, accordingly, methods of burning them have been developed.

Mainly, the methods of burning biofuels differ:

By type of biofuel;
According to the basic principles of its combustion.

To compare different combustion technologies, you need to thoroughly understand the classification of biofuel combustion technologies.

How to classify biofuel combustion technologies:

By fuel moisture;
According to the degree of fuel preparedness.

The combustion temperature of biofuel is directly related to its moisture level.

Wet biofuel combustion technologies

Wet biofuel is called wood waste - sawdust, peat, agricultural waste of animal origin.

Specifications:

This type of biofuel is the most difficult to burn;
Humidity - 31-55%;
Ash content level - high;
Particle size is not standardized.

Technologies for burning untreated (dry) biofuel

This mainly includes carpentry waste - sawdust, shavings, grain drying waste, straw, husks, etc.

Specifications:

Fuel humidity - up to 30%;
Ash content and size do not have clearly defined standards.

Refined biofuel combustion technologies

Refined biofuels are fuel pellets (including torrefied waste) and briquettes (fuel pucks, quarters), as well as fuel dust.

Characteristics of this category:

Minimum fuel moisture (about 10%);
Minimum ash content (about 2%);
The sizes of fuel particles are adjusted to special standards.

For the combustion of different types of fuel, the amount of air supplied varies. In relation to the supplied air, the following methods of burning biological fuel are distinguished:

Layer combustion

Layer combustion is considered the most famous method; it has been used for quite a long time. It is used when burning large, lumpy biofuels. There are several modifications of it:

burning on a stationary inclined grate;
combustion in a fluidized bed.

Vortex combustion

Vortex combustion is one level higher on the technological ladder than layer combustion. It is used to work with fine fuels.

Modifications of the method depend on the axis of rotation of the vortex:

Combustion with a horizontal axis of rotation of the vortex;
Vertical axis combustion.

Flaring

This is the most high-tech method of burning biofuel; a mixture of air with small particles (about 0.2-0.5 mm) of biofuel is supplied to a special chamber.

How to burn refined biofuels?

Due to the fact that the refined product has undergone initial processing, it can be burned using any of the above methods. But for each subtype of fuel, a special combustion technology has been developed.

Granules, pellets and washers are suitable for combustion in layer fireboxes. But in Europe, flaring is used to work with these types of biofuels. To do this, biomaterials must be crushed as much as possible.

Pulverized fuel, as already noted, can only be burned using the flare method. Other burning methods can be dangerous in a particular case! Both explosion and underburning of materials are possible.

Please note: some types of refined biofuels have very specific characteristics. These are granules made from straw, peat, husks and other materials. They are characterized by high ash content, in addition, their ash is capable of caking. Therefore, it is necessary to use low temperatures when burning this subtype of biofuel - up to 850 degrees.

How to burn unprepared dry biofuel?

This fine-grained type of biological fuel includes grain production and drying waste, chopped straw, husks, etc. A characteristic feature of this type is the high volatility of the particles.

The optimal combustion option would be combustion in vortex furnaces with a horizontal axis, as well as in vortex furnaces with a vertical axis. Please note: if materials are known to have high ash content, an alternative combustion method should be used.

How to burn wet biofuel?

Biofuels with a high level of humidity include sawmill and agricultural waste, peat, etc. The combustion process must begin with drying the materials.

Combustion methods:

combustion on a movable horizontal grate;
combustion on a movable inclined grate;
combustion in a fluidized bed.

What you need to know if you are going to buy a biofuel boiler.

For efficient combustion of fuel, the temperature in the firebox should not be lower than 800 degrees Celsius;
Effective combustion of wet biofuel is possible only in cases of pre-furnace;
Biofuel boilers operate efficiently only in nominal mode - 75-80% of power;
You must clearly determine the required power of the boiler you are purchasing.

The process of obtaining heat from biofuels does not always look like combustion in the real sense of the word, but rather smoldering. But the boiler, even after the fuel in the container burns out, will continue to heat the room for several hours.

Today you can find on the bio boiler market:

Burners for converting liquid fuel boilers to pellet use;
High power boiler equipment;
Industrial steam generators using biofuel;
Low-power automated boilers for private homes;
Indoor fireplaces for burning fuel pellets.

The history of biofuels goes back decades. But here’s an interesting detail: the former USSR was engaged in the creation of biofuel boilers only for the purpose of waste disposal. European countries set themselves a higher goal - to obtain the most efficient heating machine at minimal cost, and they succeeded in this!

Currently, a fairly wide range of types of biofuel boilers have been developed in Europe:

boilers using pressed biofuel - pellets and briquettes;
boilers using dry biofuel (humidity up to 30%);
boilers using wet biofuel (humidity up to 55%);
boilers for burning peat;
bark burning boilers;
boilers for burning other organic raw materials.

Modern biofuel boilers are aimed at different clients: from private consumers to large enterprises. Therefore, no matter for what purpose you are buying a biofuel boiler, you have a very wide and varied choice available to you.

Boiler houses using raw (up to 55%) and dry (up to 35%) biofuel.

Currently, a fairly wide range and type of biofuel boilers have been developed in Europe: these are boilers using pressed biofuel - pellets and briquettes (see paragraph 9), as well as dry biofuel (humidity up to 30%) and wet biofuel (humidity up to 55%).

The purpose of such boilers is very diverse: in addition to the traditional combustion of high-quality pressed biofuel (from coniferous trees) and low-quality pressed biofuel (from coniferous and deciduous trees), as well as biomass in the form of chips and sawdust, boilers have been developed for burning peat and mixtures of peat, for burning bark and bark mixtures, for burning other organic raw materials (including solid waste, garbage) and even for recycling poorly combustible raw materials.

Biofuel boilers, depending on their specific characteristics, can be targeted at a variety of market segments: from private individuals to municipal authorities, enterprises with access to raw materials or producing raw materials to manufacturing enterprises and consumers of thermal energy.

As mentioned above, the pioneers in the development of biofuel boilers were Soviet scientists, but the problem of efficient combustion of biofuel was solved by Western specialists, primarily from the countries of Northern Europe - Sweden, Finland, Denmark. They took Russian developments as a basis and brought them to perfection. This cost them huge investments, special laws to motivate the use of biofuels, and constant propaganda of environmentally friendly fuels. However, for them, as for Russia, the economy is primary. Any new equipment, and biofuel boilers are no exception, is designed to solve the main problem - to make money when replacing outdated equipment with new ones; After all, investments are made in order to make money! Investing funds to replace worn-out equipment with new ones just for the sake of replacement is immoral. In order to make money from the production of thermal energy, it is necessary to install highly efficient boilers with high efficiency, while being fully automated, requiring minimal maintenance costs, and very reliable. References to the fact that such equipment cannot be installed in timber industry enterprises are unfounded. Even in the most remote forest villages, people drive foreign cars and do not experience problems with complex equipment. You can train your own staff or enter into a service agreement.

Unfortunately, Russian manufacturers cannot yet offer such equipment for burning biofuels. Attempts to develop something effective ourselves were unsuccessful, although the efficiency is declared at the level of 90 - 95% (you can see about the efficiency here). Why hasn't it been possible to create something effective yet? Firstly, they have little knowledge of the theory of combustion of different types of biofuels. Secondly, in all developments there is some kind of know-how that cannot always be seen.

Example: in Russia there are still a lot of biofuel steam boilers of the E and DKVR brands with a Pomerantsev firebox and lighting, i.e. with an additional burner for fuel oil or diesel fuel. It is believed to be very effective. The conclusion of experts who examined such boilers sounds something like this: “This is terrifying!” And they are not exaggerating. Here is a summary of the consequences:

Thus, when burning liquid fuel and sawdust in the same chamber, slag is formed, which in turn can reduce the heat radiation of liquid fuel. The small amount of heat that is obtained as a result of radiation from burning sawdust at a low temperature is quite easy to calculate.

Thus, the above facts show that burning sawdust is a destruction of sawdust and an energy disaster if the burning occurs simultaneously with the burning of fuel oil.

The information presented above is simplified, since there are a number of other factors that have a significant impact when considering this problem..."

Since we remembered about fuel oil or diesel fuel, let's talk about the obvious difference between liquid fuel and biofuel. What is the calorific value of this fuel? But by the way, it is not the value itself (kcal/kg) that is important, but the fact that this value - the calorific value - is always constant. Therefore, the combustion process occurs automatically. What about biofuel (we are talking about uncompressed biomass here)? This value is almost always variable. Is it possible to manually control the combustion process in this case and make money by selling thermal energy? Domestic boiler manufacturers cannot yet offer a complete set of automation and control over heat release and the combustion process.

If there is no such automation, then what kind of efficiency of 90% can we talk about? And how can we talk about environmentally friendly emissions? On the contrary, incomplete combustion of biofuel leads to the release of extremely harmful substances into the atmosphere, which in the long term kill everything that grows and lives in the area of such a boiler house - first of all, this concerns forests, animals, as well as future generations of people.

But this is not the main thing. For efficient combustion of wood, it is necessary that the temperature in the entire volume of the firebox be at least 800 ° C. In the proposed domestic boilers, this is impossible in principle, because They structurally have a combustion space with water-cooled walls, which prevent uniform and sufficiently high heating of the firebox. biofuel combustion boiler room

Therefore, for now, all that remains is to buy imported boilers and wait until advanced Russian manufacturers, ZIOSAB or REMEX, for example, develop and begin producing efficient domestic boilers.

What else is important for buyers of biofuel boilers to remember?

1. It is impossible to effectively burn biofuels with humidity levels up to 30% and, even more so, above 30% without pre-furnace.
2. Biofuel boilers operate efficiently in nominal mode (75% - 80% power), just like a car, for which driving in fifth gear at a speed of 90 - 100 km/h is optimal.
3. Biofuel boilers have a lower combustion limit of 30% of maximum power. Therefore, it is important for designers to clearly determine the power of the selected boiler. The case of “more is not less” does not apply here, since this circumstance greatly affects the efficiency of the boiler.
4. And there are many other equally important nuances...

A few words about this type of biofuel, such as firewood. In some forested regions, the replacement of self-absorbing boilers with wood-burning boilers has been elevated to the rank of priority in regional heat supply policy. Many new wood-burning boilers have appeared on the market with a power of up to 2 MW or more and with a declared efficiency of 70% - 80%. And the price? ... Cheaper for nothing! Fantastic offer: very cheap boilers, no costs for wood processing, high efficiency, etc. - this is what the entire world energy industry has been dreaming about for the last 50 years. We urgently need to submit applications to the Nobel Committee. Why? Because in order to obtain 2 MW of thermal energy in 1 hour, it is necessary to burn 1.5 cubic meters. firewood of medium humidity (30%) with an efficiency of 80%. Imagine what 1.5 cubic meters is. wood:

How should combustion be organized so that this amount burns in 1 hour with an efficiency of 80%? And in 1 day you need to move 36 cubic meters. firewood How many physically strong stokers does such a boiler house need? How much firewood does such a boiler house need for the entire heating season? Here it is necessary to create a team with logging equipment. How much will the fuel cost and how much will 1 Gcal of heat produced in such a boiler house cost, which will be paid by the consumer?

But our firewood has a moisture content of 50%. We have already discussed the problems of burning materials with such humidity above. The actual efficiency of such boilers cannot exceed 30%! In order not to be unfounded, whoever has such a boiler room, please install a heat meter at the interface between the boiler room. He will calculate the heat produced by the boiler house for the heating season. Do you know how much wood was burned in this boiler room? The calorific value of firewood is 2660 kcal/kg or 1.729 Gcal/cub.m. You can easily calculate the efficiency:

Efficiency = E / Q x V,

where E is the amount of energy generated, Q is the heat of combustion of fuel and V is the volume of fuel burned in cubic meters.

The efficiency will be no more than 30%! Unfortunately, in such boiler houses there are no heat meters and consumers have to pay not for the heat received, but for the amount of heat that should have been obtained with an efficiency of 80%. Interesting? Check it out! And calculate what the real cost of 1 Gcal is for such a boiler house.

Each type of fuel has its own combustion technology, which is justified both technically and economically. Fuel pellets can be burned on various equipment. However, maximum efficiency can only be achieved with the help of boilers and burners specifically designed for this purpose.

The process of obtaining thermal energy from granules can only be called combustion at a stretch, because the granules do not burn in the literal sense of the word, but smolder. In this case, the boiler, having exhausted the fuel in the container, can continue to supply heat for 24 hours due to the low speed of the process.

In Europe, more than half of wood pellet boilers have an average power of 100 kW to 1 MW. Typically, such stoves are installed in large private homes, schools, and small businesses.

In addition to boiler rooms using pellets, there are also fireplaces using pellets and briquettes. Such fireplaces operate not as boilers, but as air heaters, and therefore do not require a piping system. More often they are used (like traditional fireplaces) as an additional means of heating.

Today, the markets of the CIS countries include burners for converting liquid fuel boilers to pellets, high-power boiler equipment, industrial biofuel steam generators, low-power automated boilers for private homes, and indoor fireplaces for burning fuel pellets. Most of the equipment is imported. However, a number of domestic enterprises offer equipment designed for burning pellets.

By the way, the first biofuel boilers appeared in Russia. Until the 60s of the twentieth century, many such boilers were developed and installed in the USSR. However, the task then was set differently: “to dispose of waste.” In the West there was a different goal: to achieve maximum efficiency in order to reduce the cost of energy produced, so the Europeans went further than the Russians in studying the nuances of burning biofuels. For example, when pine needles and a number of other elements are burned, caustic soda or sodium hydroxide hydrate is formed. The mineral salts that are formed as a result of this reaction have a detrimental effect on steel boilers, but today there are already technologies that make it possible to neutralize such harmful effects.

Each type of biofuel has its own special and specific technology. Boiler houses designed for biomass with a moisture content of less than 30% will not be effective for burning wet biofuels with a water content of about 50%, nor for refined biofuels. Wet raw materials will not burn due to the fact that they require a very high temperature inside the boiler. Wood pellets (refined biofuel) will burn in such a boiler, but at the same time they will lose economic feasibility, since the cost of a boiler using pellets is lower than that using wet or dry (up to 35%) biomass - sawdust, wood chips, etc.

Currently, a fairly wide range of types of biofuel boilers have been developed in Europe:

Boilers using pressed biofuel - pellets and briquettes,

Boilers using dry biofuel (humidity up to 30%),

Boilers using wet biofuel (humidity up to 55%),

Boilers for burning peat and peat mixtures,

Boilers for burning bark and bark mixtures,

Boilers for burning other organic raw materials.

Depending on their characteristics, boilers are aimed at different market segments: from private consumers to large enterprises and municipal boiler houses.

Fuel briquettes

The technology for producing fuel briquettes is based on the process of pressing agro-waste (sunflower husks, buckwheat, etc.) and finely ground wood waste (sawdust) with a screw under high pressure, and in some cases, when heated from 250 to 350 C°. The resulting fuel briquettes do not include any binding substances, except for one natural one - lignin, contained in the cells of plant waste. When using agricultural raw materials, it is possible to add binding elements. The temperature present during pressing contributes to the melting of the surface of the briquettes, which due to this becomes more durable, which is important for transporting the briquettes.

The raw material for the production of briquettes is the same material as for the production of pellets - sawdust of various types of wood, wood chips, sunflower husks, buckwheat, straw and many other plant wastes. The technology for producing briquettes is similar to the technology of granulation, but simpler. Briquettes come in different shapes - in the form of a brick, a cylinder or a hexagon with a hole inside. There are no standard sizes for this product.

The main factor determining the mechanical strength, water resistance and calorie content of a briquette is its density. The denser the briquette, the higher its quality indicators. The lower the density of the briquettes, the lower their calorie content. For example, with a briquette density of 650-750 kg/m3, the calorie content of the briquettes is 12-14 MJ/kg; at a density of 1200-1300 kg/m3 - 25-31 MJ/kg.

The quality of briquettes largely depends on the moisture content of the initial mixture. There are optimal and critical humidity levels. The optimal humidity is 4-10%, at which the best mechanical characteristics of briquettes are achieved (it should be borne in mind that for some types of raw materials the upper limit of humidity is 6-8%). Humidity is called critical at which the formation of briquettes is possible, but cracks appear in it - thus, the briquette does not have a marketable appearance. Critical humidity is in the range of 10-15%. At higher humidity, the resulting briquette will be “torn” by the internal moisture pressure that occurs when the crushed mass is compressed.

There are 3 main types of fuel briquettes. They differ in shape, which depends on the production method. “People” have adopted three names that come from the names of companies that produce equipment for the production of one or another briquette. Thus, RUF briquettes, NESTRO briquettes and Pini-Kay briquettes are distinguished. However, in addition to the mentioned manufacturers of briquetting equipment, there are other companies - for example, C.F.Nielsen (Denmark), UPM (Lithuania), Bogma (Sweden), Pawert-SPM AG (Switzerland), DI-PIU (Italy).

Briquettes are divided according to two principles:

The first is the raw materials from which they are made. Here they distinguish: briquettes from wood waste (shavings and sawdust without bark, waste with bark, bark, waste from MDF production, sanding dust, waste from plywood production, lignin, briquettes from agricultural waste); briquettes from agrobiomass (straw, sunflower husks, cereal husks, cotton waste, hay, reeds); briquettes from other materials (paper, cardboard, cellulose, polymers, peat).

The second is based on the pressing method and shape. Briquettes come in three types: cylindrical, extruded and brick-shaped.

Cylindrical briquettes

This type of briquettes is obtained by pressing on shock-mechanical equipment. They have an infinite length, and can be divided into both washers and logs. They have a very high density and are very popular in Europe.

Such briquettes can have not only a round, but also a square or octagonal shape, with or without a hole. The type of briquette is ordered by the buyer; it depends on which forms are more popular in each individual country. These briquettes are readily purchased by countries such as Germany, Denmark, Great Britain, Norway, Sweden, and Italy. In the domestic market, lump briquettes made using this technology are most often used as fuel for solid fuel boilers.

Extruder briquettes

These briquettes necessarily have a hole inside and a burnt top surface.

The extrusion technology for the production of briquettes is based on the process of pressing with a screw under high pressure when heated from 250 to 350 C°. The temperature present during pressing contributes to the melting of the surface of the briquettes, which thanks to this becomes durable, which is important for transporting the briquettes.

Such briquettes are placed manually into the furnace of a boiler or stove; they are in demand in the Baltic states and on the Russian domestic market.

Briquettes in the form of a brick

This product has the form of a rectangular parallelepiped with beveled corners. Such a briquette is obtained by hydraulic pressing, and its dimensions depend on the friability of the raw materials from which it is produced and the pressure exerted on it. They are well used in the domestic market, and are also well sold in all European countries.

Technology

The briquetting process is a process of compressing material under high pressure, with the release of temperature from the friction force. Due to this effect, lignin is released in the wood, which is a binder for the formation of briquettes. For briquettes not made from wood raw materials, environmentally friendly additives can be used (no more than 2%). When producing these products, you should pay special attention to moisture - a very important parameter that affects the density of the briquette. If the moisture content of the raw material exceeds 14%, the briquette falls apart into random pieces due to excess moisture.

The volume of a briquette is 1/10 of the volume of raw materials spent on its production, which provides significant savings in the transportation and storage of biofuel.

For the production of wood briquettes, piston and screw presses are used, the raw materials are sawdust and shavings. Before pressing, the material is additionally crushed and dried (humidity should not exceed 12 - 14%).

A piston press operates cyclically - with each stroke of the piston, a certain amount of material is forced through a conical nozzle; the layers corresponding to the cycles are clearly visible on the briquettes. The drive always uses a flywheel to equalize the engine load. Piston wear is small, since the relative movement between the pressed material and the piston is small, the nozzle wears out quickly. Piston presses are relatively cheap and therefore widely used.

A screw press is lighter than a piston press because there are no massive pistons or flywheels. The product comes out continuously, so it can be cut into the desired pieces. Density is higher than that of piston presses. Screw presses are less noisy due to the absence of shock loads. The disadvantages include greater energy consumption and rapid wear of the screw.

Fuel briquettes have a wide range of applications and can be used for all types of fireboxes, central heating boilers, etc. The great advantage of briquettes is the constant temperature during combustion for 4 or more hours.

Conclusion

There are barriers to using biomass as a fuel. As with fossil fuels, combustion produces CO2. However, fossil fuels emit CO2 for millions of years, creating excess CO2 in the atmosphere. In contrast, CO2 released by biomass during combustion is absorbed by plants. Biofuels are considered "carbon neutral".

In the biological equation, fossil fuels still play a key role. They are used at all stages of biomass production: plant growing, harvesting, delivery and processing. Biomass will not become carbon neutral until all stages use renewable fuels. When this will happen is a mystery to everyone. So far, biofuels can reduce CO2 emissions, since in the process of using biomass less CO2 is emitted into the atmosphere. However, in the future, biomass may replace oil, gas and coal in many areas. Governments of various countries will fund research into the development of biofuels. Among the things to be improved are biomass refineries. Such factories will accept various types of biofuels and create a constant supply for use in various industries. One refinery uses sugar in the form of cellulose and lignin from plants as the basis for fermentation, resulting in ethanol. Wood and various types of grasses can be used as biofuel. Other refineries use a thermochemical approach to standardize biomass, converting the mass into a more efficient liquid or gas. Researchers see the future of biomass as replacing petroleum as the source of many chemicals used in the modern world. Plastic items, paints and adhesives can be produced not from petroleum products, but from biomass.

References

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On this page you can get acquainted with the main aspects of the technology of burning various biofuels, including pellets and briquettes.

Biomass combustion. Biofuel boilers and boiler houses
Boilers and fireplaces using pellets and briquettes
Boiler houses using raw (up to 55%) and dry (up to 35%) biofuels
Burners for installation on liquid fuel boilers

Biofuel boiler houses - general information:

Comparison of Russian and Western Why is it generally profitable to use biomass as fuel? There are two generally accepted answers: there are economic and environmental reasons. Ecology is especially important for Western consumers, but in Russia only a few “advanced” energy producers - be it a private person heating their home, an enterprise heating their production and administrative premises, or a large municipal boiler house - think about the environment. It's a pity! However, for all consumers, the issue of economics is very important. Recent calculations and analysis of prices for main types of fuel show that biomass in many cases is superior to traditional types of fuel (not only wood or coal, but also liquid fuels - diesel and fuel oil) in terms of economics of use. Of course, in this case it is necessary to look not at the price of 1 ton of fuel, but at the cost of 1 kW of energy produced by using this fuel. We bring to your attention a comparative table of the cost of 1 kW of energy produced using various types of fuel:

As you can see, biofuel is a good alternative for those regions where there are wood reserves and the cost of wood waste is not very high due to its considerable quantity. It is especially profitable to install biofuel boiler houses at forest processing and wood processing enterprises. In addition, development trends in the Russian fuel and energy complex indicate that prices for liquid fuel and gas will constantly rise to world levels. Consequently, the use of biomass as fuel is becoming increasingly relevant for you and me.

It is very important to understand that each type of biofuel has its own special and specific technology. Boiler houses designed for biomass with a moisture content of less than 30% will not be effective for burning wet biofuels with a water content of about 50%, nor for refined biofuels. Wet raw materials will not burn due to the fact that they require a very high temperature inside the boiler, which makes no sense to achieve if drier biomass is used. Refined fuel, pellets, will burn in such a boiler, but at the same time they will lose economic feasibility, since the cost of a boiler using pellets is lower than that using wet or dry (up to 35%) biomass - sawdust, wood chips, etc. In the following sections we will briefly describe existing technologies for burning biofuels of different humidity levels.

By the way, the first biofuel boilers appeared in Russia (like many other ingenious developments). Until the 60s of the last century, many such boilers were developed and installed in the USSR. However, the economic and political situation was different then. Therefore, the task for boiler designers was set differently: “The main thing is to recycle!” Europe, on the other hand, successfully took advantage of Soviet developments in this area to solve a slightly different problem (more precisely, a radically different one): to achieve maximum efficiency in order to reduce the cost of energy produced. To do this, they studied very deeply the nature of combustion of various types of biofuels. There are a lot of nuances in burning biofuel. For example: fuel chips from the tops of coniferous trees contain pine needles. When pine needles are burned (as a result of a chain of chemical reactions), sodium alkali is formed in the firebox. There is no need to explain what sodium alkali is for steel boilers. But there are technologies that make it possible to neutralize the harmful effects - and European manufacturers also know about this.

Today in Russia, however, there are boiler manufacturers who claim that they can effectively burn almost any biofuel in a boiler designed to burn biomass with 30% moisture content. However, an analysis carried out with the involvement of foreign specialists showed that the efficiency of such boilers will be extremely low. Moreover, burning biomass in such boilers contradicts the very idea of using biomass as an environmentally friendly fuel to replace environmentally harmful fossil fuels. Harmful emissions from improper combustion of biomass are large and have a very detrimental effect on the environment, people, flora and fauna. Today, Russian producers and consumers think little about the consequences, but they will manifest themselves in the long term - on the health of future generations. If we talk about the economics of using such boilers, the situation is quite deplorable - low efficiency and immense “eating” of fuel does not lead to an increase in profits when investing in such a boiler, but to its loss. Of course, the decisive argument of the manufacturer is the cost of the design; but is it worth buying a house if you can’t live in it? In this case, indeed, “the miser pays twice,” if not more!

Boiler houses using raw (up to 55%) and dry (up to 35%) biofuels

Thus, the above facts show that burning sawdust is a destruction of sawdust and an energy disaster if the burning occurs simultaneously with the burning of fuel oil.

The information presented above is simplified, since there are a number of other factors that have a significant impact when considering this problem..."

But this is not the main thing. For efficient combustion of wood, it is necessary that the temperature in the entire volume of the firebox be at least 800 °C. In the proposed domestic boilers, this is impossible in principle, because They structurally have a combustion space with water-cooled walls, which prevent uniform and sufficiently high heating of the firebox.

Therefore, for now, all that remains is to buy imported boilers and wait until advanced Russian manufacturers, ZIOSAB or REMEX, for example, develop and begin producing efficient domestic boilers.

What else is important for buyers of biofuel boilers to remember?

1. It is impossible to effectively burn biofuels with humidity levels up to 30% and, even more so, above 30% without pre-furnace.
2. Biofuel boilers operate efficiently in nominal mode (75% - 80% power), just like a car, for which driving in fifth gear at a speed of 90 - 100 km/h is optimal.
3. Biofuel boilers have a lower combustion limit of 30% of maximum power. Therefore, it is important for designers to clearly determine the power of the selected boiler. The case of “more is not less” does not apply here, since this circumstance greatly affects the efficiency of the boiler.
4. ... And there are many other equally important nuances ...

Efficiency = E / Q x V,

Where E is the amount of energy generated, Q is the heat of combustion of fuel and V is the volume of fuel burned in cubic meters.

Boilers and fireplaces using pellets and briquettes

In Europe, on average, 50% of briquette producers and 64% of pellet producers have buyers who have medium-power boilers installed - from 100 kW to 1 MW. Typically, such stoves are installed in large private houses where many families live, as well as in schools, small businesses and official institutions. The advantage of pellet boiler houses compared to any other boiler houses in the city is a small and environmentally friendly fuel facility that can be placed even inside a building. This is not possible for either a diesel boiler house or a wet biomass boiler house.