The process of fuel combustion occurs in a stationary and fluidized bed (fluidized). In a stationary bed (Fig. 13, a) pieces of fuel do not move relative to the grate, under which the air necessary for combustion is supplied. In a fluidized bed (Fig. 13, b), particles of solid fuel move intensively relative to each other under the influence of high-speed air pressure. The fluidized bed exists within the speed limits from the beginning of fluidization to the pneumatic transport mode.

In Fig. 14 shows the structure of the fixed layer. Fuel 4, poured onto the burning coke, warms up. The released volatiles burn, forming a supra-layer flame 5. The maximum temperature (1300-1500 °C) is observed in the combustion area of ​​coke particles 3. Two zones can be distinguished in the layer: oxidative, a > 1; restorative, and< 1. В окислительной зоне продуктами реакции горючего и окислителя являются как С02, так и СО. По мере использования воздуха скорость образования С02 за­медляется, максимальное ее значение достигается при избытке воздуха а = 1. В восстановительной зоне ввиду недостаточного количества кислорода (а < 1) начинается реакция между С02 и горящим коксом (углеродом) с образованием СО. Концентрация СО в продуктах сгорания возрастает, а С02 уменьшается. Длина зон в зависимости от среднего размера 6К частиц топлива следую­щая: Ьг = (2 - 4) 6К; L2 = (4 - 6) 8К. На длины зон Lx и La (в сторону их уменьшения) влияют увеличение содержания лету­чих горючих V„, уменьшение зольности Ар, рост температуры воздуха.

Since zone 2, in addition to CO, contains Hg and CH4, the appearance of which is associated with the release of volatiles, to burn them, part of the air is supplied through blowing nozzles 3 located above the layer (see Fig. 13, a). In a fluidized bed, large fractions of fuel are suspended. The fluidized bed can be high-temperature and low-temperature. Low-temperature (800-900 °C) combustion of fuel is achieved by placing a heating surface in a fluidized bed. The dynamics of the fluidized bed (according to its height hcn) - the release of gaseous components (S08, SO, Na and 02) and the change in temperature i - pre-

Rice. 13. Fuel combustion schemes in a fixed and fluidized bed:

1 - air supply; 2 - grate; 3 - blow ring

Placed in Fig. 15. Unlike a fixed bed, where the size of fuel particles reaches 100 mm, crushed coal with 6„ is burned in a fluidized bed< 25 мм. В слое содержится 5-7 % топлива (по объему). Коэффициент теплоотдачи к поверхностям, распо­ложенным в слое, довольно высок и достигает 850 кДж/(м2-ч. К)- При сжигании малозольных топлив для увеличения теплоот­дачи в слой вводят наполнители в виде инертных зернистых ма­териалов: шлак, песок, доломит. Доломит связывает оксиды серы (до 90 %), в результате чего снижается вероятность возникнове­ния низкотемпературной коррозии. Более low level gas temperatures in a fluidized bed helps reduce the formation of nitrogen oxides during combustion, the release of which into the atmosphere causes pollution environment. In addition, slagging of the screens, i.e., sticking of the mineral part of the fuel on them, is eliminated.

It should also be noted the circulating fluidized bed, a characteristic feature of which is its approach to the work

Layer in pneumatic transport mode.

A fixed bed firebox can be manual, semi-mechanical, or mechanical with a chain grate. There are fireboxes with direct (Fig. 16, a) and reverse (Fig. 16, b) grates / driven by sprockets 2. The fuel consumption supplied from hopper 3 is regulated by the installation height of gate 4 (see Fig. 16, a) or the speed of movement of the dispensers 7 (Fig. 16, b). In grates with reverse motion, fuel is supplied to the canvas by throwers 8 of the mechanical (Fig. 16, b, c) or pneumatic (Fig. 16, d) type. Small fractions of fuel burn in suspension, and large fractions burn in a layer on the grate,

Under which air is supplied 9. Warming up, ignition and combustion of the fuel occurs due to the heat transferred by radiation from the combustion products. Slag 6 with the help of slag remover 5 (Fig. 16, a) or under the influence of its own weight (Fig. 16, b) enters the slag bunker. The structure of the burning layer is shown in Fig. 16, a. Region III of coke combustion after zone II of heating the incoming fuel (zone I) is located in the central part of the grate. Restoration zone IV is also located here. The uneven degree of fuel combustion along the length of the grate leads to the need for a sectional air supply. Most of the oxidizer should be supplied to zone III, a smaller part - to the end of the coke reaction zone and a very small amount - to the zone // of fuel preparation for combustion and zone V of slag burning. This condition is met by a stepwise distribution of excess air along the length

Rice. 17. Diagram of a boiler with a fluidized bed furnace and the design of the “air distribution cap”

Lattices. Supplying the same amount of air to all sections could lead to increased excess air at the end of the grate sheet, as a result of which there will not be enough air to burn coke (curve ag) in zone III.

The main disadvantage of fireboxes with chain grates is increased heat loss from incomplete combustion of fuel. The scope of application of such gratings is limited to steam boilers with a productivity of D - 10 kg/s and fuels with a volatile yield of UD f = 20% and reduced humidity W" = 3.25%. kg/MJ.

Tonnfa with a fluidized bed is used on a boiler with a steam output of D = 75 t/h, operating on oil shale (Fig. 17). In the zone of the low-temperature fluidized bed there are superheating 8 and evaporating 9 heating surfaces. Fuel is supplied to layer 3 from above, and air is introduced from box 6 through the “bowls” (Fig. 17, b) located along the grid. Ash is removed from the layer via ash removal system 7. Small fractions of fuel burn in suspension above the layer. The transfer of heat to the evaporative surfaces 2 in the furnace U to the superheater 11 and economizer 10 occurs as in a drum boiler.

To ensure reliable circulation of the medium in the evaporation surfaces 9 located in the layer, a circulation pump 5 is used.

Fluidized bed furnaces are characterized by reduced emissions of harmful compounds such as NOx, S02, a low probability of slagging of screens, and the possibility (due to the low temperature (Gases)) of saturation of the furnace volume with heating surfaces. Under - 44

Their causes are increased incomplete combustion of fuel, high aerodynamic drag 4 grates and 3 layers, narrow range of boiler steam output control.

Efficient combustion of solid fine-grained fuel (0-20 mm) can be achieved using the principle of a fluidized (fluidized) bed, the use of which in fuel gasification, in ferrous and non-ferrous metallurgy, chemical and oil refining, construction and other industries has made it possible to dramatically intensify a number of technological processes .

A fluidized layer is characterized by a speed of primary air that exceeds the stability limit of a dense layer, but is far from reaching the speed of soaring of medium particles. Under these conditions, all particles in the layer are intensively mixed, moving oscillatory up and down, and in general the layer has a relatively clear upper boundary. A fluidized layer of solid fuel is characterized by an increased concentration in the volume of the combustion chamber, as well as an increased relative velocity in the layer w0tп, which creates favorable conditions for high-speed combustion of fuel. Unlike a dense (stationary) layer, the aerodynamic resistance of which increases according to a power law with increasing blast intensity, in a fluidized layer the resistance does not depend on this factor (Fig. 6.10, a).

At low blowing speeds, the layer remains motionless and acts as a filter. When the critical blowing speed is reached, the pressure force of the gas flow in the layer becomes equal to the gravity force of the particles. The layer begins to expand, and with a further increase in air speed, the particles begin to move. The volume of the layer increases by 1.2-1.8 times depending on the intensity of the blast, the shape and size of the particles. The resistance of the fluidized bed does not change with a change in the blast intensity, because this increases the distance between the particles, i.e., the flow area for the gas increases. If the blowing speed increases excessively, the entire layer becomes suspended and can be removed from the working chamber.

A fluidized layer, like a liquid, is characterized by a linear law of pressure drop along its height (Fig. 6.10, b). The pressure (resistance) in a fluidized bed is proportional to its height and the density of the “boiling” material. Unlike an air suspension, where the relative speed of particles and gas approaches zero, for a fluidized bed in separate
periods (when particles fall) it reaches several meters per second.

The first use of the fluidized bed principle in a combustion device was started in 1944 by the work of the Moscow Energy Institute in relation to low-reaction fine-grained fuels (AS, coke breeze), and subsequently to brown coals. Characteristic distinctive feature MPEI firebox is two-stage scheme organization of the combustion process. Fluidized bed furnaces are used as the first stage, where intense and deep thermal preparation fuel: heating, drying and release of high-temperature flammable gases. The second stage of a fluidized bed furnace is an afterburning chamber for the combustible gas emitted by the fluidized bed and the thermally prepared entrainment particles contained in it.

When such fireboxes operate on the ASH, about a third of the air required for complete combustion fuel. Gas formation in a fluidized bed (Fig. 6.11) occurs similar to gas formation in a dense layer, however, the oxygen and reduction zones have increased thicknesses. The temperature of the fluidized bed is maintained at a level that prevents melting of the ash, in order to avoid slagging of the layer. This can be achieved by installing cooling surfaces in a layer, recirculating flue gases and etc.
In a normally operating fluidized bed, molten slag does not occur.

A relatively high and fairly uniform temperature along the height of the bed (when operating at an ash burner is about 1000 °C), favorable hydrodynamic conditions determined by the increased relative gas velocity, and the presence of a sufficiently developed oxidation surface of fine-grained fuel ensure high productivity of the fluidized bed as the first stage of a semi-gas furnace with boiling water. layer. Under the conditions under consideration, the flammable gas emerging from the layer has a temperature of about 1000°C and a heat of combustion of 1.7-2.5 MJ/m 3 . Apparent Density heat flow on the blast grate is q n =4.7/7 MW/m 2.

The second stage of a fluidized bed furnace for gas afterburning and removal can be done according to various options. In Fig. 6.12 shows the layout of a single-chamber fluidized bed furnace with a hot water boiler; The second stage of the fluidized bed furnace is located directly above the bed. In production conditions, such a firebox operated on coke breeze and coal from the Moscow region. Fuel size 0-20 mm. Thermal power hot water heating boiler about 5 MW. When working with coke breeze (Ar = 17.33%; Wp = 19.85%), about 30% of the total amount of air required for combustion enters under the grate, which has live section 3-4%. The rest of the air is supplied to combustion chamber above the fluidized bed through two rows of tuyeres. The necessary cooling of the fluidized fuel bed to implement the slag-free mode (1000°C) is achieved by water-cooled surfaces located in the fluidized bed and included in the boiler circulation system.

The heat transfer coefficient from the fluidized bed to the cooling surface is about 250-400 W/(m 2 *K). It was also provided for the injection of water directly into the fluidized bed to be able to regulate its temperature if necessary. When working on BM grade coal near Moscow (A p = 19.8%; W p = 33.84%), about 50-60% of the total air was supplied to the layer, the temperature of the layer was maintained at 900 °C. Maintaining the desired ash content of the layer, which prevents its extinction and ensures small losses from mechanical underburning with sumping, is carried out by continuous or periodic “blowing” of the layer through cesspool. The height of the layer in the boiling state is maintained at 600-800 mm. The required air pressure under the grate is 3400-3900 Pa. When working on coke breeze, fuel consumption is about 0.3 kg/s, and when working on coal near Moscow - 0.5 kg/s. In this case, the apparent heat flux density of the blast grate was q H - 4.8 MW/m 2 with a volumetric heat release density for the entire combustion chamber g y = 0.17 MW/m.

In Fig. Figure 6.13 shows some other options for the second stage of a fluidized bed furnace. In diagram a - a variant of a single-chamber furnace with a fluidized bed, where secondary air is supplied tangentially to intensify combustion of exhaust air; in diagram b, the coupling of the first and second stages of the Double-Chamber Race is carried out using a special turbulent burner; In diagram c, a cyclone chamber with liquid slag removal is used as the second stage of a fluidized bed furnace. In the 50s, three fluidized bed furnaces were built and were in operation in the USSR, which showed the possibility efficient combustion various fine-grained fuels.

Particular interest in organizing the combustion of fuels in a fluidized bed is caused by a number of circumstances. Various fuels can be used for combustion, including low-grade fuels with a particle size of 0-20 mm. At the same time, energy costs for fuel preparation are significantly reduced. The location of heating surfaces in a fluidized bed, where the heat transfer coefficient is 200-300 W/(m 2 *K), provides a significant reduction in the metal consumption of the installation. Working with a relatively low-temperature layer (800-1000°C) leads to a significant reduction in atmospheric pollution with sulfur compounds, since most of it remains in the layer and is removed along with the ash. To increase the degree of sulfur capture, lime or dolomite can be added to the fluidized bed. Due to the low temperature, the gases leaving the fluidized bed contain virtually no nitrogen oxides. The sublimation of alkaline compounds in fuel ash is also reduced, which leads to reduced contamination of heating surfaces.

The schematic diagram of a boiler with a fluidized bed furnace with part of the heating surfaces placed in the bed is shown in Fig. 6.14. Provision is made for the return to the furnace of the captured carry-over from the fluidized bed, which usually contains a significant amount of unburned carbon. A scheme with afterburning of the exhaust in a special device is also possible. Currently, various fluidized bed furnaces are in operation abroad and in the USSR, including those for boilers with high steam output, as well as those operating under pressure (up to 1 MPa), which leads to further intensification of the combustion process of solid fuels and improvement of technical and economic indicators.

Along with the combustion of solid fuel in a fluidized bed, highly efficient combustion of gas and liquid fuels can be organized. To do this, a fluidized layer of inert material(sand, brick chips etc.) in which gas is burned or liquid fuel. Boiler heating surfaces can also be installed in such a fluidized bed, which intensifies heat transfer.

Technological furnaces with a fluidized bed are also widely used in industry, in particular for firing various sulfur-containing materials (pyrites, copper and zinc concentrates, etc.). To maintain the bed temperature at a level that prevents slagging, cooled elements are used that are located in the fluidized bed and remove excess heat. These elements usually produce steam. For more information about such energy technology installations, see Chapter. 18.

Status of consideration of the project by the Coordination Council: Not considered. Implementation objects: Industry, Boiler houses, RTS, CTS, CHP. Effect of implementation:
- for object saving on capital investments for the construction of stations up to 10%, saving fuel, increasing the efficiency of boiler units;
- For municipality reducing fuel consumption, improving the quality and reliability of heat sources, reducing tariffs for consumers. .

Stationary fluidized bed boiler- a stationary boiler for burning fuel in a fluidized layer of inert material, ash or mixtures with part of the heating surfaces placed in this layer.

fluidized bed- fluidized bed, granular bed state bulk material, in which, under the influence of a flow of gas or liquid (liquefying agents) passing through it, particles of solid material intensively move one relative to another. In this state, the layer resembles a boiling liquid, acquiring some of its properties, and its behavior obeys the laws of hydrostatics. In K. s. close contact is achieved between the granular material and the liquefying agent, which makes effective application K. s. in chemical industry apparatus where interaction between solid and fluid phases is necessary (diffusion, catalytic processes, etc.).

Table according to JSC NPO TsKTI data

			Quantity	Thermal power, MW		Year of commissioning
	village Pussi, Estonia, AS "Repo" company				ancient waste/shale
	village Jüri, Estonia, AS "ELVESO"	new hot water boiler			cutters peat/ ancient waste
	village Kietaviškės, Lithuania, DOMINGA HARDWOOD AB (together with Kazlu Rudos Metalas JSC)				ancient waste
	Marijampole, Lithuania, "Marijampoles RK" (together with JSC "Kazlu Rudos Metalas")	(equipping a new boiler with an NTKS firebox)			ancient waste
	Maksatikha village, Tver region, Maksatikhinsky DOK	(reconstruction of existing boiler)			ancient waste
		(equipping a new boiler with an NTKS firebox)			ancient waste
	Plunge, Lithuania, AB "PLUNGES BIOENERGIJA" (together with JSC "Kazlu Rudos Metalas")	(equipping a new boiler with an NTKS firebox)			ancient waste
	Vileika, Belarus, Mini-CHP based on RK No. 3 (together with Axis Industries JSC)	New steam boiler D=22 t/h, p=24 bar, t=350ºC			ancient waste	V this moment- carrying out commissioning work
	V.Sinyachikha village, Sverdlovsk region, plywood mill ZAO "Fankom"	New steam boiler Ep-20-2.4-350 DF			ancient waste	Currently - equipment manufacturing stage

The main features of fuel combustion inherent only to the fluidized bed are:

Intensive mixing of fuel particles with gas bubbles, allowing to avoid the appearance of significant temperature imbalances in the layer and, as a consequence, slagging;

Intensification of heat transfer from the fluidized bed to heat transfer surfaces (particle hard material, cooling at the surface of the pipe washed by the working fluid, due to the difference in densities, gives off several orders of magnitude more heat than a gas particle of the same volume cooling to the same temperature; the heat transfer coefficient to pipes immersed in a fluidized bed is ~250 W/m2K in modern furnaces);

Intensification of combustion of solid fuel (explained by an increase in the specific surface area of ​​oxidation and the constant “renewal” of its surface, due to intense pulsation, rotation, collisions, crushing and abrasion into fine dust).

In fluidized bed furnaces (Fig. 1, 2), fine coal of brown and bituminous coals with piece sizes from 2 to 12 mm are burned.

The temperature of the layer, in order to avoid slagging, is regulated by introducing steam in an amount of 0.3-0.6 kg/kg. It is possible to replace steam with water sprayed using spray guns (water consumption 0.2-0.3 kg/kg).

The disadvantages of fluidized bed furnaces are:

Carbon removal is up to 20-30% of the total fuel carbon (therefore, these fireboxes are recommended to be used if afterburning is possible, 0-1 mm of carryover in the working space of the boiler);

Slagging of the internozzle space and the nozzles themselves of the air distribution grates with insufficient dynamic air pressure;

Abrasive wear of heat transfer surfaces, especially high for those immersed in a fluidized bed.

Rice. 1. Semi-gas fluidized bed furnace.

1 - fuel bunker; 2 - screw feeder; 3 - grate; 4 - blow box for supplying primary air; 5 - secondary air supply; 6 - hopper shutter; 7 - blower fan.

Rice. 2. Fluidized bed furnace with submerged heat exchanger.

1 - air distribution grille; 2 - heat exchanger; 3 - slag feeder; 4 - pilot burner; 5 - device for accumulating and removing ash; 6 - screw conveyor.

In order to add description energy saving technology to the Catalog, fill out the questionnaire and send it to marked “to Catalog”.

Combustion of fuels in a fluidized bed

Modern development energy and the worsening environmental situation in the world required the search and development of more progressive and environmentally friendly clean technologies combustion of solid fuels.

One of the promising directions that ensures the environmental friendliness of the use of solid low-grade fuels in power plants of the future should be considered their combustion in boilers with fluidized bed furnaces of various modifications: classical, circulating, aerospouting using aerospouting devices, since this significantly reduces emissions of SO 2 and NO x is already at the combustion stage.

1.1. Combustion of solid fuels in boiler furnaces with a classic fluidized bed

Rice. 1.1. Schemes of installations with a fluidized bed: a – classic fluidized bed: b – circulating fluidized bed; c – fluidized bed under pressure; 1 – main air; 2 – fuel supply; 3 – secondary air; 4 – ash output; 5 – return of entrainment; 6 – combustion products; 7 – cyclone; 8 – heating surface; 9 – turbine and compressor

In Fig. 1.1. A diagram of a furnace with a classic bubble fluidized bed is shown. In a bubbly fluidized bed at atmospheric pressure coal (or other solid fuel) is burned in a bed of solid particles (usually limestone), which is fluidized by combustion air supplied underneath the bed. The layer is heated with hot air or gases using a special gas burner. Fluidized bed boilers are designed so that the bed temperature is in the range of 815–870 o C. Possibility of operation at low temperatures leads to several benefits. Due to the low temperature for binding SO 2 can be used as a sorbent inexpensive materials, such as limestone and dolomite. When limestone or dolomite is added to a layer, the reaction between CaO and SO 2 produces CaSO 4 . Depending on the sulfur content of the fuel and the amount of sorbent, SO 2 emissions can be reduced by 90% or more. Thermal nitrogen oxides are formed at temperatures above 1300 o C. As the temperature decreases, the reaction rate of NO x formation decreases greatly. At temperatures of 815–870 o C, the amount of NO x formed in the fluidized bed is significantly less than in traditional boiler plants operating at higher temperatures.

Fluidized bed combustion (FBC) technology has whole line advantages compared to pulverized coal combustion of solid fuels.

These include:

– simplicity of design;

– possibility of burning low-quality coals;

– safety in operation;

– absence of fine grinding mills;

– binding of SO 2 and SO 3;

– suppression of NO x (up to 200 mg/m3).

Due to intense mixing, the temperature is equalized throughout the fluidized bed, so the layer can be considered isothermal. Heating surfaces lowered into a fluidized bed have a very high heat transfer coefficient. This is facilitated by the destruction of the boundary layer on the heat transfer surface, as well as direct contact of particles with the heat removal surface.

The disadvantages of this combustion technology include abrasive wear of heating surfaces located in the layer; high values ​​of mechanical underburning, limiting the power of boiler units equipped with fluidized bed furnaces to 250 t/h. More powerful boilers require larger grates, which creates difficulties in ensuring uniform blast speed.

The ideal fuel for fluidized bed boilers is oil shale, which has high reactivity and high ash content, which determines the large mass of the material, and therefore the combustion temperature is stabilized, rapid drying of the fuel and good burnout occur.

When using low-ash Kansk-Achinsk coals, a large addition of inert material is required. The combustion of coals with a high content of alkali metal salts is very beneficial when used in fluidized bed furnaces, when practically no evaporation of salts occurs. This gives rise to the possibility of involving so-called “salty” coals in the energy sector.

An example of this is the industrial experience of introducing a fluidized bed for burning slag “salty” coals in the USA.

In 1986, Babcock-Wilcox converted the mechanically fired boiler at the Montana-Dakota Thermal Power Plant to a bubbly fluidized bed unit. This boiler was originally designed to produce 81.9 kg/s (295 t/h) of steam at a pressure of 9 MPa and a temperature of 510 o C for burning brown coal from the Belakh deposit.

However, the high content of sodium compounds in the fly ash led to severe slagging of the furnace and contamination of the superheater. Before reconstruction with a fluidized bed device, the power was limited to 50 MW with a design capacity of 72 MW. In order to avoid slagging and contamination of heating surfaces and ensure operation at full power, a fluidized bed was used. New installation with a fluidized bed with a cross-section of 12.2 x 7.9 m was built into an old boiler with minimal changes to the pressure surfaces of the screens. The air distribution grille and its surrounding walls were cooled with water. A superheater and an evaporator were placed in the layer to provide the necessary steam production and steam superheating and limit the layer temperature at 815 o C. The gas velocity in the layer was 3.7 m/s, and the depth of the layer in operating condition was 1.37 m. To turn on and When the installation was launched, the air supply was carried out through eight sections. Since brown coal from the Belakh deposit is a highly reactive fuel, there was no provision for the return of fly ash. Taking into account the low sulfur content and high alkaline content of the fuel, sand was used as the layer material. The boiler was put into operation in May 1987. Now this unit carries a load of 80 MW without slagging or surface contamination. NOx concentrations measured were 0.14 g/MJ.

Ph.D. A.M. Sidorov, director,
Ph.D. A. A. Scriabin, Deputy Director for Science,
A.I.Medvedev, technical director,
F.V. Shcherbakov, chief engineer,
Research Center PA "Biyskenergomash", Barnaul, Altai Territory

On the feasibility of using furnaces with a forced low-temperature fluidized bed

A promising direction in the development of industrial and municipal energy is the introduction of highly efficient schemes for organizing the combustion process in a forced low-temperature fluidized bed (FFL). This technology ensures stable combustion in the volume of the layer and in the space above the layer. It allows the combustion of almost any type of fuel and combustible waste at a relatively low temperature (800-1000 °C) without sintering the layer.

Fireboxes with a classic bubble fluidized bed are characterized by low liquefaction rates and, accordingly, not very high thermal stresses of the air distribution grille (up to 3 MW/m2). The processes are carried out in the volume of the layer. Combustion above the layer quickly stops due to the rapid cooling of the flue gases, so all the blast must be supplied under the layer. The area above the layer and combustion screens are used with low efficiency; excess heat from the layer must be removed by heating surfaces immersed in it. As a result, fireboxes with a classic layer have large area and bulky. In addition, the operation of submerged surfaces is accompanied by intense abrasive wear. Despite the low level of bed temperatures, even a short-term cessation of liquefaction or a local increase in temperature is dangerous due to sintering of bed particles. This predetermines a narrow control range.

The main difference between FKS and other types of fluidized bed is the high (3-10 m/s) liquefaction speed - bed forcing. In this case, low mechanical underburning (less than 1.5-2.5%) is ensured due to the expansion of the cross-section of the combustion space above the layer towards the top. This promotes the return of large particles to the layer (recirculation) and reduces the removal of small particles. FKS does not have heating surfaces immersed in the layer and associated problems. Reliable performance screen pipes in the zone

the dynamic effect of the layer is ensured by the use effective means protection against abrasive wear.

Forced air distribution grille provides the following advantages:

• ■ provides small dimensions of the grid and fluidized bed reactor and, therefore, favorable conditions for modernization and reconstruction installed equipment, low cost and low repair costs;
• ■ allows you to burn fuel of coarser crushing compared to the classic fluidized bed; actually for brown coals maximum size a piece can reach 30-50 mm;
• ■ provides more reliable operation layer according to the conditions of occurrence, and, therefore, expands the range of load control.

FKS technology implies operation of the layer in fuel gasification mode at actual values ​​of excess air α<1,0. Величина избытка определяется калорийностью и видом топлива и может составлять 0,3-0,7 (для бурых углей больше). Это позволяет еще более уменьшить габариты реактора и снизить затраты на подачу воздуха под решетку. Высвободившийся воздух увеличивает долю вторичного дутья, необходимого для дожигания уноса и продуктов газификации, - до 70%, что позволяет организовать активное вихревое движение топочных газов, способствующее повышению эффективности сгорания топлива. Теплонапряжение воздухораспределительной решетки в расчете на поданное топливо может достигать 10-15 МВт/м2.

The FKS technology for forcing the air distribution grille is close to the circulating fluidized bed (CFB) and has the following advantages:

■ the ability to integrate FKS boilers into standard boiler cells;

■ absence of slagging of heating surfaces;

■ good performance of FKS fireboxes, compared to mechanized layer fireboxes, in terms of cost, service life, reliability and maintainability;

■ lack of mill equipment;

■ the ability to burn a wide range of fuels and combustible waste;

■ wide possibilities for regulating the operating parameters of FKS boilers and high stability of load bearing, which allows them to be used in conjunction with steam turbines;

■ high environmental performance in terms of emissions of sulfur and nitrogen oxides.

At the same time, compared to CFB, the introduction of forced fluidized bed technology requires significantly lower capital costs.

Particularly attractive options for implementing FCS are those associated with the reconstruction of boiler houses. They allow you to save and use most of the installed equipment, significantly reduce capital costs and, therefore, are affordable for most industrial energy and utility companies. At the same time, the invested funds quickly pay off and profitability increases.

Typically, the basis for introducing FCC technology is:

■ new construction with the ability to work on low-grade coal;

■ the need to ensure reliable heat and energy supplies (for example, by replacing fuel, expanding the range of coal used, using local low-grade fuels or combustible waste);

■ the need to reduce fuel costs by replacing it with cheaper ones, or by increasing the efficiency of its combustion;

■ the need to replace outdated, worn-out equipment;

■ the need to dispose of combustible waste, such as waste from coal preparation, timber and wood processing, slag from layer boilers, etc.

Experience in operating boilers with FKS

To date, we, together with a number of enterprises, have implemented furnaces with FCS at more than 50 facilities. As examples, we will give, in our opinion, the most interesting of them.

Example 1. Reconstruction of the Chita CHPP-2 with the conversion of layer boilers to combustion of Kharanorsky coal in a fluidized bed. In the period 1999-2003. Using FKS technology, a complete reconstruction of the Chita CHPP-2 was carried out with the transfer of TS-35 layer boilers to the combustion of Khara Norsk brown coal (Qrn = 2720 kcal/kg; Ap = 13.2%; Wр = 40%) in a fluidized bed.

The need for reconstruction was caused by the low efficiency of layer boilers and significant repair costs. In addition, the task was set to increase the boiler productivity to 42 t/h.

The reconstruction affected the following boiler components:

■ the profile of the lower part of the firebox has been changed. The chain grille has been removed, the front and rear screens have been extended downwards. The side walls are covered with heavy lining at a height from the air distribution grille to the axis of the cooling panels; the screens of the side walls remain unchanged;

■ removable air distribution caps are installed on the air distribution grille, ensuring uniform liquefaction of the layer, and two pipes for draining the layer, cooled with water, to remove slag;

■ to light the boiler, a kindling device is installed in a separate air box under the grate. Hot gases generated during the combustion of diesel fuel heat the layer from below and ensure ignition of the coal supplied to the furnace. After stable ignition of coal in the layer, the kindling device is turned off;

■ sharp blast nozzles are installed on the front and rear walls of the firebox. Air, preheated in the air heater, is supplied to the nozzles by a standard VD-13.5×1000 fan;

■ to ensure liquefaction of the layer, two VDN-8.5-I×3000 high-pressure fans were additionally installed;

■ the second superheater package along the gas flow, located in the rotary gas duct, has been enlarged;

■ the second air heater cube along the gas flow was dismantled;

■ the boiler economizer is increased by 3.5 loops;

■ the blades of the standard D-15.5 smoke exhauster were enlarged, and the engine was replaced with a more powerful one, which is associated with an increase in boiler productivity from 35 to 42 t/h.

The reconstructed furnace with FKS is fundamentally different from traditional fluidized bed furnaces, namely:

■ high liquefaction speed (up to 9-10 m/s), kakutopox CFB. Due to intensive mixing, there are no uneven temperatures and fuel concentrations over the layer area. The layer material is partially carried into the furnace volume and, being intensively cooled, flows down the back screen back into the layer, cooling it. Due to repeated intra-furnace circulation of the layer material, good combustion of combustibles is ensured;

■ only 50-60% of the air involved in combustion is supplied under the grate; the rest of the air is supplied through secondary blast nozzles. Lack of air in the layer leads to partial gasification of the fuel and two-stage combustion;

■ secondary air supplied through nozzles located on the front and rear walls of the furnace forms a powerful horizontal vortex, which contributes to the afterburning of gases and carried-out fines.

The applied technical solutions made it possible to significantly improve the boiler’s performance, in particular:

■ increase fuel burnout without the use of expensive separation devices and entrainment return used in CFB boilers. Maximum losses with mechanical underburning do not exceed 2.5%;

■ expand the temperature control limit of superheated steam due to intensified heat exchange in the furnace caused by a horizontal vortex;

■ regulate the layer temperature by changing the air flow under the grate without the use of submerged heating surfaces. When switching to gasification mode, the temperature of the layer decreases. The dependence of the layer temperature on the air flow rate under the grate has a clearly expressed maximum at the point of their stoichiometric ratio; with an increase or decrease in air in the layer, the temperature drops. Thanks to this, the boiler has no load restrictions due to the high bed temperature;

■ achieve moderate wear of convective surfaces, because 60-70% of the total entrainment is the slippage of relatively large particles (100-1000 microns) that did not fall into the horizontal vortex, the rest is very fine ash, which has little effect on wear;

■ reduce emissions of nitrogen oxides by 2 times (relative to layer and flare furnaces). Due to two-stage combustion and low bed temperatures throughout the entire control range of loads and with any excess air in the furnace, the maximum NOx concentration does not exceed 200 mg/m3;

■ exclude significant losses due to chemical underburning. The concentration of carbon monoxide due to afterburning in a vertical vortex does not exceed 100 ppm.

Comparative characteristics of the station No. 7 boiler before and after reconstruction are given in Table 1.

Table 1. Characteristics of the boiler Art. No. 7 Chitinskaya CHPP-2.

Parameter name	Meaning
	Before reconstruction	After reconstruction
Productivity, t/h	35	42
Steam pressure, MPa	3,8	3,8
Steam temperature, °C	440	440
Feedwater temperature, °C	105	105
Heat loss with mechanical underburning, %	4,5	2,5
Gross boiler efficiency, %	82	86
Load control range, %	40-100	52-100
Excess air behind the firebox	1,4	1,3
Flue gas temperature, °C	175	180
CO concentration (no more), mg/m3	4000	100
NOX concentration (no more), mg/m3	450	200

The results of adjustment tests showed that the maximum steam output of the boiler after reconstruction is limited by the productivity of the smoke exhauster and amounts to 44 t/h. The filling of the furnace at loads above 35-38 t/h improves, the content of carbon monoxide in the gases decreases.

According to operating data, the combustion mode of the reconstructed boilers is characterized by high stability. Deviations in the temperature of superheated steam in stationary mode are short-term and do not exceed ±5 °C. Temperature imbalances across the width of the furnace and pulsations are not observed. The operating temperature of the layer is 820-980 °C.

During commissioning tests, it was revealed that the minimum thermal loads that ensure self-heating of the layer fully satisfy the specified boiler firing schedule. Coal consumption to maintain the minimum bed temperature is approximately 1.5 t/h, which is about 15% of fuel consumption for the boiler at rated load.

Ignition of the boiler begins with diesel fuel. After stable combustion of coal in the layer at a temperature of 500-550 °C, the pilot nozzle is turned off, the minimum fuel consumption is set, and heating of the boiler continues without outside interference in the combustion mode. Diesel fuel consumption for heating the layer during kindling from the cold reserve is no more than 200 liters. After the boiler has been idle for less than 6 hours, diesel fuel consumption is halved. When the boiler is idle for less than 3 hours, kindling is carried out without the use of liquid fuel, while the coal is ignited from the heat accumulated in the layer. Fuel oil can be used instead of diesel fuel.

Thus, as a result of the reconstruction, it was possible to obtain a more reliable and controllable boiler with a gross efficiency of at least 4% higher than before the reconstruction. The reliability, safety and environmental characteristics of the new firebox are not only comparable to layer and torch fireboxes, but also superior to them.

To prevent abrasive wear of heating surfaces in contact with the fluidized bed, the technology of surfacing pipes with wear-resistant material was used at the Chita CHPP-2 (Fig. 1).

Considering the simplicity of the design and the possibility of burning any low-grade fuel, the new combustion device may be suitable for the design and reconstruction of pulverized coal and gas-oil boilers of low and medium power. Converting boilers to burning coal using this technology will not only save liquid fuel for kindling, but also eliminate the consumption of fuel oil for lighting the torch. The share of fuel oil used for these purposes can be reduced by an order of magnitude.

Example 2. Construction of a boiler house with three boilers with FKS furnaces. In 2003, the Amuragrocenter OJSC company constructed a boiler house with three KE-10-14-225S boilers for burning a mixture of brown coal (80%) and oat husks (20%) with FKS furnaces.

In Fig. Figure 2 shows the installation of equipment on pre-prepared foundations of building structures of the boiler building, which is a light metal frame with pre-manufactured sandwich-type wall panels. Experience in the construction of boiler houses of this design shows the possibility of reducing the full cycle of construction of boiler houses with a thermal capacity of 15-30 Gcal/h in 5-6 months, excluding stripping operations.

Example 3. Construction of a boiler house with three steam boilers for burning brown coal from the Itat deposit. In 2005, the management of OJSC Altaivagon (Rubtsovsk, Altai Territory) decided to build its own boiler house with three KE-25-14-225PS steam boilers (Fig. 3), dictated by economic considerations. As a result of construction, the enterprise received its own energy source, equipped with highly efficient boilers made using FKS technology, with an efficiency of 84-87%, burning cheap brown coal from the Itatskoe deposit (characteristics of coal for working mass: pH = 3100 kcal/kg; Wр = 39%; Ar =12%).

To increase the reliability and durability of the heating screen surfaces in the fluidized bed action zone, two methods were used to protect pipes from abrasive wear (Fig. 4). At a height of 1 m from the air distribution grille, cast iron linings (grade ChH16, hardness 400-450 HV, operating temperature up to 900 °C) are fixed on the pipes; at a height of 1 m from the linings, protection is applied by gas spraying of a layer of self-fluxing alloy PR-NH17SR4-40/100 ( thickness of the deposited layer - from 0.5 to 1.4 mm, hardness - 418 HV). As operating experience shows, this protection guarantees reliable operation of screen pipes.

The boiler diagram KE-25-14-225PS is shown in Fig. 5.

The boiler is equipped with an automatic control system that provides all standard adjustments, protections and alarms for low and medium power boilers. Provides start-up of the boiler from a cold state and a “hot” standby and operation of the boiler in automatic mode.

The boiler KE-25-14-225PS, in accordance with the requirements of SNiP and the operating technology of the furnace, is equipped with a measurement system that provides control and recording of the following parameters:

■ level (height) of the layer (control);

■ water level in the drum (water flow through the boiler) (control and registration);

■ steam pressure in the drum (water pressure at the inlet and outlet of the boiler) (control);

■ air pressure in the air distribution grille (control);

■ vacuum in the furnace (control);

■ vacuum at the smoke exhauster (control);

■ flue gas temperature (control);

■ layer temperature (control and registration);

■ temperature of combustion gases (control);

■ temperature of water leaving the boiler in hot water mode (control and registration);

■ steam consumption (control and registration).

The control and monitoring panel is shown in Fig. 6.

All automation systems are combined into one control circuit. The operator's (boiler operator) workplace is located in a separate room. It can simultaneously control several boilers and other process equipment.

Table 2. Results of testing the operation of the KE-25-14-225PS boiler st. No. 3 of the Altaivagon boiler house, Rubtsovsk.

Table 3. Results of industrial tests of boilers KV-F-11.63-115PS st. No. 1, 2 and 3 in the central boiler house in Borzya.

Characteristics	Art. No. 1		Art. No. 2		4,6	10,1	4,9	9,5	4,2	9,8
Water consumption, m3/h	218	218	210	210	200	200
CO concentration, mg/nm3 (a=1.4)	405	360	180	382	477	438
NOX concentration, mg/nm3 (oc=1.4)	347	353	235	409	297	207
Combustible content in entrainment, %	10	14,5	15,8	15,5	11,9	13
Air flow per layer, Nm3/h	7200	13410	6900	13760	8210	12940
Total air flow per boiler, Nm3/h	10000	20600	11000	22400	12000	20600
Fluidized bed temperature, °C	765	810	726	792	742	792
Gross boiler efficiency, %	89,9	84,4	86,3	84,3	84,6	83,5
Specific consumption of standard fuel, kg/Gcal	155,1	155,8	158,9	161,9	160,2	161,3

Note: fuel - brown coal: 0^=3012 kcal/kg; Ar=13.2%; Wp=35.9%.

Control and monitoring is carried out from a computer from a separate room via a network, or from a touch screen on the control panel. The boiler control panel view is shown in Fig. 7.

The test results of the KE-25-14-225PS boiler (Table 2) showed high efficiency, low emissions of NOx (300-385 mg/nm3) and CO (80-300 mg/nm3). The content of combustibles in the entrainment with increasing load from 30 to 100% changed in the range of 10-21% with a corresponding change in mechanical combustion from 1.59 to 3.87%. The boiler efficiency varied within the entire load range from 84.9 to 86.3%. The steam temperature was 204-225 °C. The temperature of the fluidized bed averaged 890 °C and ensured reliable slag-free operation of the boiler. The specific consumption of equivalent fuel was 188.3 kg/MW.

Example 4. Reconstruction of a boiler house by replacing worn-out boilers with two hot water boilers with FKS fireboxes. In 2005-2006 In the city of Mogocha, Trans-Baikal Territory, the housing and communal services boiler house was reconstructed by replacing worn-out boilers with two water-heating boilers KEV-10-95PS (Fig. 8) with FKS furnaces for burning Kharanorsky brown coal.

Main technical characteristics of the boiler:

■ heating capacity 6.98 MW (6 Gcal/h);

■ water pressure at the inlet is no more than 0.8 MPa (8.0 kgf/cm2);

■ water pressure at the outlet is not less than 0.24 MPa (2.4 kgf/cm2);

■ outlet water temperature no more than 95 °C;

■ boiler efficiency (gross) 85.87%;

■ total fuel consumption 2596 kg/h. A design feature of the boiler is the presence of an FKS firebox installed in the lower part of the boiler combustion chamber, formed by brick walls converging to the bottom. The FKS firebox consists of an air distribution grille (area - 2.4 m2) with an air box at the bottom, a kindling chamber with a nozzle, a layer drain pipe and a slag removal device. Removable cast iron caps are installed on the grille in a corridor order. Air is supplied under the grille from a high-pressure fan VDN 8.5×3000-I (17,000 m3/h; 75 kW).

The fuel preparation system provides coal with a particle size of up to 25-30 mm into the layer. Feeding is carried out into the layer by two PTL 600 feeders with dismantled rotors.

Before lighting the boiler, inert filler is loaded onto the air distribution grille. Sand, small crushed stone or slag of fractions 1-6 mm are used as inert filler. The height of the poured layer is 250-350 mm.

The boiler firing system includes a solar oil tank, a fuel pump, mechanical and fine filters, and fittings. The boiler is fired by heating the layer with hot gases supplied under the grate, which are formed during the combustion of liquid fuel in the ignition chamber. The temperature of the layer during kindling is controlled by changing the consumption of kindling fuel.

To reduce losses due to mechanical underburning, the boiler is equipped with a two-stage entrainment return system. The first stage operates by expanding the furnace upward, which makes it possible to separate the largest particles flying out of the layer. Along the inclined walls of the lower part of the furnace, the particles roll back into the volume of the fluidized bed. The second stage is the convective beam of the boiler. The flammable particles trapped in it are returned through pneumatic transport lines to the above-layer space.

The boiler has two-stage combustion. Part of the air (about 70%) enters under the air distribution grille. The remaining air is fed into the combustion chamber through sharp blast nozzles. Both primary and secondary air are supplied from one VDN 8.5×3000-I fan.

A smoke exhauster DN-12.5× 1500 (75 kW) is installed behind the boiler.

Currently, the installed boilers are in operation, the staff reviews are positive.

Example 5. Reconstruction of a central boiler house by installing three station boilers with an FKS firebox. In 2006, in the city of Borzya, the central boiler house was reconstructed with the installation of three new hot water boilers KV-F-11.63-115PS, station No. 1, 2 and 3. The boiler diagram is shown in Fig. 9.

Main design characteristics of the boiler:

■ heating capacity 11.63 MW (10 Gcal/h)

■ water pressure at the inlet no more than 1.0 MPa (10.1 kgf/cm2);

■ hydraulic resistance of the boiler unit is 0.18 MPa (1.8 kgf/cm2);

■ inlet water temperature is at least 70 °C;

■ outlet water temperature no more than 115 °C;

■ boiler plant efficiency (gross) 84%;

■ estimated fuel consumption (Kharanor brown coal) 4112 kg/h.

The results of industrial tests of new boilers are given in table. 3.

Example 6. Construction of a pilot industrial energy-technological installation for the production of semi-coke from Berezovsky brown coal using an FKS reactor. In 2006, in the boiler house of OJSC Razrez Berezovsky 1, a pilot industrial energy-technological installation for the production of semi-coke from Berezovsky brown coal was put into operation (Qrn = 16168 kJ/kg, Ap = 2.93%, Wр = 34.1%) with maintaining the thermal power of the boiler.

The installation is designed on the basis of a serial water heating boiler KV-TS-20. A special feature of the installation is the use of an FKS reactor.

Coal from the bunker is fed into the fluidized bed through four chutes located at the front of the boiler. In the reactor at temperatures of 580-700 °C, its pyrolysis is carried out, accompanied by the combustion of volatiles and fines removed from the layer. Air is supplied under the reactor grate from a high-pressure fan VDN-8.5×3000.

From the reactor, the resulting charcoal is “overflowed” into a tubular cooler.

Cooled there to a temperature of 100-120 °C, it is transported to a storage hopper using a conveyor system.

As a result of thermochemical treatment of coal in a fluidized bed reactor, semi-coke is obtained (Qrn = 27251-27774 kJ/kg, Ap = 7.95-8.25%, Wр = 4.2-3.42%).

The weight yield of semi-coke is about 25% of the coal consumption supplied to the boiler.

The energy-technological installation operates with optimal ratios of primary and secondary air and supplied fuel, which allows, with minimal heat losses and harmful emissions for this design, to obtain 20 Gcal/h of heat and ensure a stable output of semi-coke of the required quality with good economic indicators. The estimated payback period for investment costs is no more than 17.5 months.