Physical properties of flue gases. Heat capacity of products of complete combustion in a stoichiometric volume of air. List of used literature

When constructing a furnace, ideally, one would like to have such a design that would automatically give as much air as needed for combustion. At first glance, this can be done with chimney. Indeed, the more intensely the firewood burns, the more hot flue gases, the more thrust should be (carburetor model). But it's not. The draft does not depend at all on the amount of hot flue gases generated. Draft is the pressure drop in the pipe from the pipe head to the firebox. It is determined by the height of the pipe and the temperature of the flue gases, or rather, their density.

Thrust is determined by the formula:

F \u003d A (p in - p d) h

where F is the thrust, A is the coefficient, p in is the density of the outside air, p d is the flue gas density, h is the height of the pipe

Flue gas density is calculated by the formula:

p d \u003d p in (273 + t in) / (273 + t d)

where t in and t d - temperature in degrees Celsius of the outside atmospheric air outside the pipe and flue gases in the pipe.

Velocity of flue gases in the pipe (volume flow, i.e. suction capacity of the pipe) G does not depend on the height of the pipe at all and is determined by the temperature difference between the flue gases and the outside air, as well as the area cross section chimney. A number of practical conclusions follow from this.

Firstly, the chimneys are made high not at all in order to increase the air flow through the firebox, but only to increase the draft (that is, the pressure drop in the pipe). This is very important to prevent tipping of the draft (furnace smoking) in case of wind pressure (the thrust value must always exceed the possible wind pressure).

Secondly, it is convenient to regulate the air flow with the help of devices that change the area of ​​\u200b\u200bthe free section of the pipe, that is, with the help of valves. With an increase in the cross-sectional area of ​​the chimney channel, for example, by a factor of two, one can expect an approximately twofold increase volume flow air through the firebox.

Let's make it simple and good example. We have two identical ovens. We combine them into one. We get a double-sized stove with double the amount of burning wood, with double the air flow and the cross-sectional area of ​​\u200b\u200bthe pipe. Or (which is the same thing), if more and more firewood flares up in the firebox, then it is necessary to open the valves on the pipe more and more.

Thirdly If the stove burns normally in the steady state, and we additionally let a stream of cold air into the firebox past the burning wood into the chimney, then the flue gases will immediately cool down, and the air flow through the stove will decrease. At the same time, burning firewood will begin to fade. That is, we do not seem to directly affect the firewood and direct the additional flow past the firewood, but it turns out that the pipe can pass less flue gases than before, when this additional air flow was absent. The pipe itself will reduce the air flow to the firewood that was previously, and besides, it will not let in an additional flow of cold air. In other words, the chimney will be blocked.

That is why cold air leaks through cracks in the chimneys, excessive air flows in the firebox, and indeed any heat loss in the chimney leading to a decrease in the temperature of the flue gases are so harmful.

Fourth, the greater the coefficient of gas-dynamic resistance of the chimney, the lower the air flow. That is, it is desirable to make the walls of the chimney as smooth as possible, without turbulences and without turns.

Fifth, the lower the temperature of the flue gases, the more sharply the air flow changes with fluctuations in the temperature of the flue gases, which explains the situation of the instability of the pipe when the furnace is ignited.

At sixth, at high flue gas temperatures, the air flow rate is independent of the flue gas temperature. That is, with a strong heating of the furnace, the air flow ceases to increase and begins to depend only on the cross section of the pipe.

Instability issues arise not only when analyzing the thermal characteristics of a pipe, but also when considering the dynamics of gas flows in a pipe. Indeed, the chimney is a well filled with light flue gas. If this light flue gas does not rise very quickly, then there is a possibility that the heavy outside air can simply sink into the light gas and create a falling downward flow in the pipe. This situation is especially likely when the walls of the chimney are cold, that is, during the ignition of the furnace.

Rice. 1. Scheme of the movement of gases in a cold chimney: 1 - firebox; 2 - air supply through the blower; 3-chimney; 4 - valve; 5 - chimney tooth; 6-flue gases; 7-failing cold air; 8 - air flow causing thrust tipping.

a) smooth open vertical pipe
b) a pipe with a valve and a tooth
c) pipe with top valve

Solid arrows show the directions of movement of light hot flue gases. Dashed arrows show the directions of downward flows of cold heavy air from the atmosphere.

On the rice. 1a a furnace is schematically shown, into which air 2 is supplied and flue gases 6 are removed through the chimney. even the firebox. This falling flow can replace the “regular” air flow through blower 2. Even if the stove is locked with all the doors and all air intake dampers are closed, the stove can still burn due to the air coming from above. By the way, this is what often happens when coal burns out with the oven doors closed. A complete tipping of the draft may even occur: air will enter from above through the pipe, and flue gases will exit through the door.

In reality, on the inner wall of the chimney there are always bumps, growths, roughness, upon collision with which the flue gases and the oncoming downward cold air flows swirl and mix with each other. At the same time, the cold downward air flow is pushed out or, heating up, begins to rise upwards mixed with hot gases.

The effect of turning downward flows of cold air upwards is enhanced in the presence of partially open dampers, as well as the so-called tooth, which is widely used in the technology of manufacturing fireplaces ( rice. 1b). The tooth prevents the flow of cold air from the chimney into the fireplace space and thereby prevents the fireplace from smoking.

Downdrafts of air in the pipe are especially dangerous in foggy weather: flue gases are not able to evaporate the smallest droplets of water, they cool down, the thrust decreases and may even tip over. At the same time, the stove smokes heavily, does not flare up.

For the same reason, stoves with damp chimneys smoke a lot. Top gate valves are particularly effective in preventing downflows ( rice. 1v), adjustable depending on the speed of the flue gases in the chimney. However, the operation of such valves is inconvenient.

Rice. Fig. 2. Dependence of the excess air coefficient a on the time of heating the furnace (solid curve). The dotted curve is the required air consumption G consumption for the complete oxidation of the combustion products of firewood (including soot and volatile substances) in flue gases (in relative units). The dashed-dotted curve is the actual air consumption G of the pipe provided by the pipe draft (in relative units). The excess air coefficient is the quotient of G pipe separation per G flow

A stable and sufficiently strong draft occurs only after the walls of the chimney have warmed up, which takes a long time, so there is always not enough air at the beginning of the heating. In this case, the excess air coefficient is less than unity, and the furnace smokes ( rice. 2). And vice versa: at the end of the heating, the chimney remains hot, the draft remains for a long time, although the firewood has almost burned out (the excess air coefficient is more than one). Metal furnaces with metal insulated chimneys reach the regime faster due to their low heat capacity compared to brick chimneys.

The analysis of the processes in the chimney can be continued, but it is already clear that no matter how good the stove itself is, all its advantages can be reduced to zero by a bad chimney. Of course, ideally, the chimney would need to be replaced modern system forced extraction of flue gases by means of an electric fan with adjustable flow rate and with pre-condensation of moisture from flue gases. Such a system, among other things, could clean the flue gases from soot, carbon monoxide and other harmful impurities, as well as cool the discharged flue gases and provide heat recovery.

But all this is in the distant future. For a summer resident and gardener, a chimney can sometimes become much more expensive than the stove itself, especially in the case of heating a multi-level house. Sauna chimneys are usually simpler and shorter, but the heat output level of the stove can be very high. Such pipes, as a rule, are very hot along the entire length, sparks and ash often fly out of them, but condensation and soot are insignificant.

If for now you plan to use the sauna building only as a bathhouse, then the pipe can also be made uninsulated. If you also think about the bathhouse as a place of possible stay (temporary residence, overnight stays), especially in winter, then it is more expedient to immediately make the pipe insulated, and qualitatively, “for life”. At the same time, stoves can be changed at least every day, the design can be selected more conveniently and more appropriately, and the pipe will be the same.

At least if the oven is on long burning(smoldering firewood), then pipe insulation is absolutely necessary, because at low power (1 - 5 kW) an uninsulated metal pipe will become completely cold, condensate will flow abundantly, which in the most severe frosts can even freeze and block the pipe with ice. This is especially dangerous in the presence of a spark arresting grid and umbrellas with small passage gaps. Spark arresters are useful for intensive heating in summer and extremely dangerous for weak firewood burning conditions in winter. Due to the possible clogging of pipes with ice, the installation of deflectors and umbrellas on chimneys was banned in 1991 (and on chimneys gas ovens even earlier).

For the same reasons, you should not get carried away with the height of the pipe - the level of thrust is not so important for a non-return sauna stove. If it smokes, you can always quickly ventilate the room. But the height above the roof ridge (at least 0.5 m) must be observed to prevent tipping of the thrust during gusts of wind. On flat roofs, the pipe should protrude above the snow cover. In any case, it is better to have a lower pipe, but warmer (than higher, but colder). Tall chimneys are always cold and dangerous in winter.

Cold chimneys have a lot of disadvantages. At the same time, non-insulated, but not very long pipes on metal stoves warm up quickly during kindling (much faster than brick pipes), remain hot with vigorous heating, and therefore are very widely used in baths (and not only in baths), especially since they are relatively cheap. Asbestos-cement pipes are not used on metal furnaces, since they are heavy, and also collapse when overheated with fragments flying.

Rice. 3. The simplest designs of metal chimneys: 1 - a metal round chimney; 2 - spark catcher; 3 - a cap to protect the pipe from atmospheric precipitation; 4 - rafters; 5 - roof sheathing; 6 - wooden blocks between rafters (or beams) to design a fire opening (cutting) in the roof or ceiling (if necessary); 7 - roof ridge; eight - soft roof(roofing material, hydrostekloizol, soft tiles, corrugated cardboard-bitumen sheets, etc.); 9 - metal sheet for roofing and covering the opening (it is allowed to use flat sheet aceida - asbestos-cement electrical insulating board); 10 - metal drainage pad; 11 - asbestos sealing of the gap (joint); 12 - metal cap-otter; 13 - ceiling beams(with filling the space with insulation); 14 - ceiling lining; 15 - attic floor (if necessary); 16 - metal sheet of ceiling cutting; 17 - metal reinforcing corners; 18 - metal cover of the ceiling cut (if necessary); 19 - non-combustible heat-resistant insulation (expanded clay, sand, perlite, mineral wool); 20 - protective pad (metal sheet over a layer of asbestos cardboard 8 mm thick); 21 - metal pipe screen.

a) non-thermal insulated pipe;
b) a heat-insulated shielded pipe with a heat transfer resistance of at least 0.3 m 2 -deg / W (which is equivalent to a brick thickness of 130 mm or a mineral wool insulation thickness of 20 mm).

On the rice. 3 shows typical wiring diagrams for non-insulated metal pipes. The pipe itself should be purchased from stainless steel with a thickness of at least 0.7 mm. The most popular diameter of the Russian pipe is 120 mm, the Finnish one is 115 mm.

According to GOST 9817-95, the cross-sectional area of ​​​​a multi-turn chimney should be at least 8 cm 2 per 1 kW of rated heat output released in the furnace when wood is burned. This power should not be confused with the thermal power of a heat-intensive furnace, released from the outer brick surface of the furnace into the room according to SNiP 2.04.05-91. This is one of our many misunderstandings. normative documents. Since heat-intensive furnaces are usually heated only 2-3 hours a day, the power in the furnace is about ten times greater than the power of heat release from the surface of a brick oven.

Next time we will talk about the features of the installation of chimneys.

When fuel carbon is burned in air according to the equation (21C + 2102 + 79N2 = 21C02 + 79N2), for each volume of CO2 in the combustion products there are 79: 21 = 3.76 volumes of N2.

Combustion of anthracite, lean coal and other fuels with a high carbon content produces combustion products that are similar in composition to carbon combustion products. When hydrogen is burned according to the equation

42H2+2102+79N2=42H20+79N2

For each volume of H20, there are 79:42 = 1.88 volumes of nitrogen.

In the combustion products of natural, liquefied and coke oven gases, liquid fuels, firewood, peat, brown coal, long-flame and gas coal and other fuels with a significant hydrogen content in the combustible mass, a large number of water vapor, sometimes exceeding the volume of CO2. The presence of moisture in the top

Live, naturally, increases the content of water vapor in the combustion products.

The composition of the products of complete combustion of the main types of fuel in a stoichiometric volume of air is given in Table. 34. From the data in this table it can be seen that the N2 content in the combustion products of all types of fuel significantly exceeds the total content of C02-f-H20, and in the carbon combustion products it is 79%.

Hydrogen combustion products contain 65% N2; combustion products of natural and liquefied gases, gasoline, fuel oil and other hydrocarbon fuels contain 70-74% N2.

Rice. 5. Volumetric heat capacity

Combustion products

4 - carbon combustion products

5 - hydrogen combustion products

The average heat capacity of the products of complete combustion that do not contain oxygen can be calculated by the formula

C \u003d 0.01 (Cc02C02 + Cso2S02 + Cn20H20 + CN2N2) kcal / (m3 - ° С), (VI. 1)

Where Сс0г, Cso2, СНа0, CNa are the volumetric heat capacities of carbon dioxide, sulfur dioxide, water vapor and nitrogen, and С02, S02, Н20 and N2 are the content of the corresponding components in the combustion products, % (vol.).

In accordance with this formula (VI. 1) takes the following form:

C \u003d 0.01. (Cc02 /? 02 + CHj0H20-bCNi! N2) kcal / (m3 "°C). (VI.2)

The average volumetric heat capacity of CO2, H20 and N2 in the temperature range from 0 to 2500 °C is given in Table. 36. Curves characterizing the change in the average volumetric heat capacity of these gases with increasing temperature are shown in fig. 5.

From the table. 16 data and curves depicted in fig. 5 shows the following:

1. The volumetric heat capacity of CO2 significantly exceeds the heat capacity of H20, which, in turn, exceeds the heat capacity of N2 over the entire temperature range from 0 to 2000 °C.

2. The heat capacity of CO2 increases with increasing temperature faster than the heat capacity of H20, and the heat capacity of H20 faster than the heat capacity of N2. However, despite this, the weighted average volumetric heat capacities of carbon and hydrogen combustion products in a stoichiometric volume of air differ little.

This situation, somewhat unexpected at first glance, is due to the fact that in the products of complete combustion of carbon in air for each cubic meter CO2, which has the highest volumetric heat capacity, accounts for 3.76 m3 of N2 with the minimum volumetric

Heat capacity, and in the products of hydrogen combustion for each cubic meter of water vapor, the volumetric heat capacity of which is less than that of CO2, but more than that of N2, there is half the amount of nitrogen (1.88 m3).

As a result, the average volumetric heat capacities of the combustion products of carbon and hydrogen in air are equalized, as can be seen from the data in Table. 37 and comparison of curves 4 and 5 in Figs. 5. The difference in the weighted average heat capacities of the combustion products of carbon and hydrogen in air does not exceed 2%. Naturally, the heat capacities of the combustion products of fuel, which consists mainly of carbon and hydrogen, in a stoichiometric volume of air lie in a narrow region between curves 4 and 5 (shaded in Fig. 5).

Products of complete combustion of various vidoges; fuels in stoichiometric air in the temperature range from 0 to 2100 °C have the following heat capacity, kcal/(m3>°C):

Fluctuations in heat capacity of combustion products various kinds fuels are relatively small. At solid fuel with a high moisture content (firewood, peat, brown coal, etc.) the heat capacity of combustion products in the same temperature range is higher than that of fuel with a low moisture content (anthracite, coal, fuel oil, natural gas, etc.) . This is due to the fact that during the combustion of fuel with a high moisture content in the combustion products, the content of water vapor increases, which has a higher heat capacity compared to diatomic gas - nitrogen.

In table. 38 shows the average volumetric heat capacities of the products of complete combustion, not diluted with air, for various temperature ranges.

Table 38

An increase in the moisture content in the fuel increases the heat capacity of the combustion products due to an increase in the content of water vapor in them in the same temperature range, compared with the heat capacity of the combustion products of fuel with a lower moisture content, and at the same time lowers the combustion temperature of the fuel due to an increase in the volume of combustion products due to water pair.

With an increase in the moisture content in the fuel, the volumetric heat capacity of the combustion products increases in a given temperature range and, at the same time, the temperature interval decreases from 0 to £max due to a decrease in the value<тах. ПОСКОЛЬКУ ТЄПЛОЄМКОСТЬ ГЭЗОВ уМвНЬ — шается с понижением температуры, теплоемкость продуктов сгорания топлива с различной влажностью в интервале температур от нуля до <тах для данного топлива претерпевает незначительные колебания (табл. 39). В соответствии с этим можно принять теплоемкость про­дуктов сгорания всех видов твердого топлива от 0 до tmax равной 0,405, жидкого топлива 0,401, природного, доменного и генераторного газов 0,400 ккал/(м3-°С).

This makes it possible to considerably simplify the determination of the calorimetric and calculated combustion temperatures (according to the method described in Chapter VII). The error allowed in this case usually does not exceed 1%, or 20 °.

From the consideration of curves 4 and 5 in Figs. 5 it can be seen that the ratio of the heat capacity of the products of complete combustion of carbon in a stoichiometric volume of air in the temperature range from 0 to t ° C, for example, from 0 to

200 and from 0 to 2100 °C are practically equal to the ratio of the heat capacities of hydrogen combustion products in the same temperature ranges. The specified ratio of heat capacities C' remains practically constant for the products of complete combustion of various types of fuel in a stoichiometric volume of air.

In table. 40 shows the ratios of the heat capacities of the products of complete combustion of fuel with a low content of ballast, which passes into gaseous combustion products (anthracite, coke, coal, liquid fuel, natural, petroleum, coke oven gases, etc.) in the temperature range from 0 to t ° С and in the temperature range from 0 to 2100 °C. Since the heat capacity of these types of fuel is close to 2100 ° C, the indicated ratio of heat capacities C' is equal to the ratio of heat capacities in the temperature range from 0 to t and from 0 to tm&x-

In table. 40 also shows the values ​​of C', calculated for the products of combustion of fuel with a high content of ballast, which passes during the combustion of fuel into gaseous combustion products, i.e. moisture in solid fuel, nitrogen and carbon dioxide in gaseous fuel. The heat capacity of these types of fuel (wood, peat, brown coal, mixed generator, air and blast-furnace gases) is 1600-1700 °C.

Table 40

As can be seen from Table. 40, the values ​​of C' and K differ little even for fuel combustion products with different ballast content and heat output.

Heat of combustion. The net calorific value of dry gaseous fuel Qf varies widely from 4 to 47 MJ / m3 and depends on its composition - the ratio and quality of combustible and non-combustible

components. The lowest Qf value is for blast-furnace gas, the average composition of which is about 30% combustible gases (mainly carbon monoxide CO) and about 60% non-combustible nitrogen N2. Greatest

The Qf value for associated gases, the composition of which is characterized by a high content of heavy hydrocarbons. The heat of combustion of natural gases fluctuates in a narrow range Qf = 35.5…37.5 MJ/m3.

The lower calorific value of individual gases that make up gaseous fuels is given in Table. 3.2. See Section 3 for methods for determining the calorific value of gaseous fuels.

Density. There are absolute and relative density of gases.

The absolute gas density rg, kg/m3, is the mass of gas per 1 m3 of the volume occupied by this gas. When calculating the density of an individual gas, the volume of its kilomo-la is taken equal to 22.41 m3 (as for an ideal gas).

Relative gas density Rotn is the ratio of absolute gas density under normal conditions and similar air density:

Rotn \u003d Rg / Pv \u003d Rg / 1.293, (6.1)

Where rg, pE are, respectively, the absolute density of gas and air under normal conditions, kg / m3. The relative density of gases is usually used to compare different gases with each other.

The values ​​of the absolute and relative density of simple gases are given in Table. 6.1.

The density of the gas mixture pjM, kg/m3, is determined based on the additivity rule, according to which the properties of gases are summarized according to their volume fraction in the mixture:

Where Xj is the volumetric content of the 7th gas in the fuel, %; (rg); - density of the j-th gas, which is part of the fuel, kg/m3; n is the number of individual gases in the fuel.

The values ​​of the density of gaseous fuels are given in table. P.5.

Gas density p, kg/m3, depending on temperature and pressure, can be calculated by the formula

Where p0 is the gas density under normal conditions (T0 = 273 K and p0 = 101.3 kPa), kg/m3; p and T are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K.

Almost all types of gaseous fuels are lighter than air, therefore, when leaking, gas accumulates under the ceilings. For safety reasons, before starting the boiler, it is imperative to check the absence of gas in the most likely places of its accumulation.

The viscosity of gases increases with increasing temperature. The values ​​of the dynamic viscosity coefficient p, Pa-s, can be calculated using the empirical Seser-Land equation

Table 6.1

Characteristics of the components of gas fuel (at t - O ° C chr \u003d 101.3 kPa)

Where p0 is the coefficient of dynamic viscosity of the gas under normal conditions (G0 = 273 K and p0 - 101.3 kPa), Pa-s; T is the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.

For a mixture of gases, the coefficient of dynamic viscosity can be approximately determined from the values ​​of the viscosity of the individual components:

Where gj is the mass fraction of the j-th gas in the fuel,%; Zu - coefficient of dynamic viscosity of the j-th component, Pa-s; n is the number of individual gases in the fuel.

In practice, the coefficient of kinematic viscosity V, m2/s, is widely used, which
which is related to the dynamic viscosity p through the density p by the dependence

V = r / r. (6.6)

Taking into account (6.4) and (6.6), the coefficient of kinematic viscosity v, m2/s, depending on pressure and temperature, can be calculated by the formula

Where v0 is the coefficient of kinematic viscosity of the gas under normal conditions (Go = 273 K and p0 = 101.3 kPa), m2/s; p and G are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.

The values ​​of the coefficients of kinematic viscosity for gaseous fuels are given in Table. P.9.

Table 6.2

Viscosity and thermal conductivity coefficients of gas fuel components

(at t \u003d 0 ° С ir \u003d 101.3 kPa)

Thermal conductivity. Molecular energy transfer in gases is characterized by the coefficient of thermal conductivity ‘k, W / (m-K). The thermal conductivity coefficient is inversely proportional to pressure and increases with increasing temperature. The values ​​of the X coefficient can be calculated using the Sutherland formula

Where X,0 is the thermal conductivity of the gas under normal conditions (G0 = 273 K and Po = 101.3 kPa), W / (m-K); p and T are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.

The values ​​of thermal conductivity coefficients for gaseous fuels are given in Table. P.9.

The heat capacity of gaseous fuel per 1 m3 of dry gas depends on its composition and is generally defined as

4L=0.01(CH2H2+Ccos0 +

CCH4CH4 + cCo2cOg + - + cx. X;), (6.9) - heat capacities of the constituent components of the fuel, respectively, hydrogen, carbon monoxide, methane, carbon dioxide and /-th component, kJ / (m3-K); H2, CO, CH4, CO2, ..., Xg--

The heat capacities of the combustible components of gaseous fuel are given in Table. P.6, non-combustible - in table. P.7.

Heat capacity of wet gaseous fuel

Cgtl, kJ/(m3-K), is defined as

<тл = ctrn + 0,00124cHzq йтля, (6.10) где drTn- влагосодержание газообразного топлива,

Explosiveness. A mixture of combustible gas with air in certain proportions in the presence of fire or even a spark can explode, i.e., it ignites and burns at a speed close to the speed of sound propagation. Explosive concentrations of combustible gas in air depend on the chemical composition and properties of the gas. Volume concentration ignition limits for individual combustible gases in a mixture with air are given earlier in Table. 6.1. Hydrogen (4.. .74% by volume) and carbon monoxide (12.5...74%) have the widest ignition limits. For natural gas, the average lower and upper flammability limits are 4.5 and 17% by volume, respectively; for coke - 5.6 and 31%; for domain - 35 and 74%.

Toxicity. Toxicity is understood as the ability of a gas to cause poisoning of living organisms. The degree of toxicity depends on the type of gas and its concentration. The most dangerous gas components in this respect are carbon monoxide CO and hydrogen sulfide H2S.

The toxicity of gas mixtures is mainly determined by the concentration of the most toxic of the components present in the mixture, while its harmful effect, as a rule, is markedly enhanced in the presence of other harmful gases.

The presence and concentration of harmful gases in the air can be determined by a special device - a gas analyzer.

Almost all natural gases are odorless. To detect a gas leak and take safety measures, natural gas is odorized before it enters the main, that is, it is saturated with a substance that has a pungent odor (for example, mercaptans).

The heat of combustion of various types of fuel varies widely. For fuel oil, for example, it is over 40 MJ/kg, and for blast-furnace gas and some grades of oil shale, it is about 4 MJ/kg. The composition of energy fuels also varies widely. Thus, the same qualitative characteristics, depending on the type and brand of fuel, can sharply differ quantitatively from each other.

The given characteristics of the fuel. For comparative analysis, in the role of characteristics summarizing the quality of the fuel, the given characteristics of the fuel are used, %-kg / MJ, which are generally calculated by the formula

Where хг is an indicator of the quality of working fuel, %; Q[ - specific heat of combustion (lowest), MJ/kg.

So, for example, to calculate the reduced

Humidity ash content of sulfur S „p and

Nitrogen N^p (for fuel operating condition)

Formula (7.1) takes the following form, %-kg/MJ:

TOC o "1-3" h z KP=Kl GT; (7.2)

4f=l7e[; (7.3)

snp=S’/Єї; (7.4)

^p=N7 Q[. (7.5)

As an illustrative example, the following comparison is indicative, provided that different fuels are burned in boilers of the same thermal power. So, a comparison of the reduced moisture content of coal near Moscow

Grades 2B (WЈp = 3.72% -kg / MJ) and Nazarov-

Coal 2B (W^p = 3.04%-kg / MJ) shows that in the first case, the amount of moisture introduced into the furnace of the boiler with fuel will be approximately 1.2 times greater than in the second, despite the fact that that the working humidity of coal near Moscow (W[ \u003d 31%) is less than that of

Nazarovsky coal (Wf = 39%).

conditional fuel. In the energy sector, in order to compare the efficiency of fuel use in various boiler plants, to plan the extraction and consumption of fuel in economic calculations, the concept of conventional fuel has been introduced. As standard fuel, such fuel is accepted, the specific calorific value (lowest) of which in working condition is equal to Qy T = 29300 kJ/kg (or

7000 kcal/kg).

For each natural fuel, there is a so-called dimensionless thermal equivalent E, which can be greater or less than unity:

The thermophysical properties of gaseous combustion products necessary for calculating the dependence of various parameters on the temperature of a given gaseous medium can be established on the basis of the values ​​given in the table. In particular, these dependences for the heat capacity are obtained in the form:

C psm = a -1/ d,

where a = 1,3615803; b = 7,0065648; c = 0,0053034712; d = 20,761095;

C psm = a + bT sm + cT 2 sm,

where a = 0,94426057; b = 0,00035133267; c = -0,0000000539.

The first dependence is preferable in terms of approximation accuracy, the second dependence can be taken to carry out calculations of lower accuracy.

Physical parameters of flue gases
(at P = 0.0981 MPa; R CO2 = 0.13; p H2O = 0.11; R N2 = 0.76)

APPENDIX 3

(reference)

Air and smoke permeability of air ducts and valves

1. To determine leaks or air leaks in relation to the ventilation ducts of anti-smoke systems, the following formulas can be used, obtained by approximating tabular data:

for class H air ducts (in the pressure range 0.2 - 1.4 kPa): ΔL = a(R - b)With, where ΔL- suctions (leaks) of air, m 3 / m 2 h; R- pressure, kPa; a = 10,752331; b = 0,0069397038; With = 0,66419906;

for class P air ducts (in the pressure range 0.2 - 5.0 kPa): where a = 0,00913545; b=-3.1647682 10 8 ; c =-1.2724412 10 9 ; d= 0,68424233.

2. For normally closed fire dampers, the numerical values ​​of the specific characteristic of resistance to smoke and gas permeation depending on the gas temperature correspond to the data obtained during bench fire tests of various products at the experimental base of VNIIPO: