Chimney, calculation. Thermal properties of flue gases Heat capacity of flue gases at different temperatures table
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. Lowest value Qf 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 summed up respectively volume fraction in the mix:
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)
|
Chemical |
Molar mass M, |
Density |
Bulk concentrates |
||
|
Name of gas |
Absolute |
Relative |
Zionic limits of ignition of gas in a mixture with air,% |
||
|
combustible gases |
|||||
|
Propylene |
|||||
|
carbon monoxide |
|||||
|
hydrogen sulfide |
|||||
|
non-flammable gases |
|||||
|
Carbon dioxide |
|||||
|
sulphur dioxide |
|||||
|
Oxygen |
|||||
|
Atmospheric air. |
|||||
|
water vapor |
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)
|
Name of gas |
Viscosity factor |
Thermal conductivity coefficient N03, W/(m-K) |
Sutherland coefficient C, K |
|
|
Dynamic r-106, Pa-s |
Kinematic v-106, m2/s |
|||
|
combustible gases |
||||
|
Propylene |
||||
|
carbon monoxide |
||||
|
hydrogen sulfide |
||||
|
non-flammable gases Carbon dioxide |
||||
|
Oxygen |
||||
|
Atmospheric air |
||||
|
Water vapor at 100 °C |
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).
Heat of combustion various kinds 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 production 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:
State educational institution of higher professional education
"Samara State Technical University"
Department of Chemical Technology and Industrial Ecology
COURSE WORK
in the discipline "Technical thermodynamics and heat engineering"
Topic: Calculation of the installation for the recovery of heat from the waste gases of a process furnace
Completed by: Student Ryabinina E.A.
ZF course III group 19
Checked by: Consultant Churkina A.Yu.
Samara 2010
Introduction
Most chemical enterprises generate high- and low-temperature thermal waste, which can be used as secondary energy resources (SER). These include flue gases from various boilers and process furnaces, cooled streams, cooling water and exhaust steam.
Thermal VER to a large extent cover the heat demand of individual industries. Thus, in the nitrogen industry, more than 26% of the heat demand is met by means of VER, in the soda industry - more than 11%.
The number of used HORs depends on three factors: the temperature of the HORs, their thermal power and the continuity of the output.
At present, the most widespread is the utilization of heat from industrial waste gases, which have a high temperature potential for almost all fire engineering processes and can be used continuously in most industries. Waste gas heat is the main component of the energy balance. It is used mainly for technological, and in some cases - for energy purposes (in waste heat boilers).
However, the widespread use of high-temperature thermal VERs is associated with the development of methods for utilization, including the heat of hot slags, products, etc., new methods for utilizing the heat of exhaust gases, as well as with the improvement of the designs of existing utilization equipment.
1. Description of the technological scheme
In tube furnaces without a convection chamber, or in radiant-convection type furnaces, but having a relatively high initial temperature of the heated product, the flue gas temperature can be relatively high, which leads to increased heat loss, reduced furnace efficiency and higher fuel consumption. Therefore, it is necessary to use the heat of the waste gases. This can be achieved either by using an air heater that heats the air entering the furnace for fuel combustion, or by installing waste heat boilers that make it possible to obtain water vapor necessary for technological needs.
However, for the implementation of air heating, additional costs are required for the construction of an air heater, blowers, as well as additional power consumption consumed by the blower motor.
To ensure the normal operation of the air heater, it is important to prevent the possibility of corrosion of its surface from the side of the flue gas flow. This phenomenon is possible when the temperature of the heat exchange surface is lower than the dew point temperature; at the same time, part of the flue gases, directly in contact with the surface of the air heater, is significantly cooled, the water vapor contained in them partially condenses and, absorbing sulfur dioxide from the gases, forms an aggressive weak acid.
The dew point corresponds to the temperature at which the saturated vapor pressure of water is equal to the partial pressure of water vapor contained in flue gases.
One of the most reliable ways to protect against corrosion is to preheat the air in some way (for example, in water or steam heaters) to a temperature above the dew point. Such corrosion can also occur on the surface of convection pipes if the temperature of the raw material entering the furnace is below the dew point.
The source of heat to increase the temperature of saturated steam is the oxidation reaction (combustion) of the primary fuel. The flue gases formed during combustion give off their heat in the radiation and then convection chambers to the raw material flow (steam). Superheated water vapor enters the consumer, and the combustion products leave the furnace and enter the waste heat boiler. At the outlet of the KU, saturated water vapor is fed back to the steam superheating furnace, and the flue gases, being cooled by the feed water, enter the air heater. From the air heater, the flue gases enter the KTAN, where the water flowing through the coil is heated and goes directly to the consumer, and the flue gases are released into the atmosphere.
2. Furnace calculation
2.1 Calculation of the combustion process
Let's determine the lower calorific value of fuel combustion Q R n. If the fuel is an individual hydrocarbon, then its calorific value Q R n equal to the standard heat of combustion minus the heat of vaporization of water in the combustion products. It can also be calculated from the standard thermal effects of the formation of initial and final products based on the Hess law.
For a fuel consisting of a mixture of hydrocarbons, the calorific value is determined according to the additivity rule:
where Q pi n- heat of combustion i-th fuel component;
y i- concentration i-th fuel component in fractions of a unit, then:
Q R n cm = 35.84 ∙ 0.987 + 63.80 ∙ 0.0033+ 91.32 ∙ 0.0012+ 118.73 ∙ 0.0004 + 146.10 ∙ 0.0001 \u003d 35.75 MJ / m 3.
Molar mass of fuel:
M m = Σ M i ∙ y i ,
where M i- molar mass i-th fuel component, from here:
M m = 16.042 ∙ 0.987 + 30.07 ∙ 0.0033 + 44.094 ∙ 0.0012 + 58.120 ∙ 0.0004 + 72.15 ∙ 0.0001 + 44.010 ∙ 0.001+ 28.01 ∙ 0.002 = 28.01 ∙ 0.002
kg / m 3,
then Q R n cm, expressed in MJ/kg, is equal to:
MJ/kg.
The calculation results are summarized in Table. one:
Fuel composition Table 1
Let us determine the elemental composition of the fuel, % (mass):
,
where n i C , n i H , n i N , n i O- the number of carbon, hydrogen, nitrogen and oxygen atoms in the molecules of individual components that make up the fuel;
The content of each component of the fuel, wt. %;
x i- the content of each component of the fuel, they say. %;
M i is the molar mass of the individual fuel components;
M m is the molar mass of the fuel.
Composition check :
C + H + O + N = 74.0 + 24.6 + 0.2 + 1.2 = 100% (mass).
Let us determine the theoretical amount of air required to burn 1 kg of fuel; it is determined from the stoichiometric equation of the combustion reaction and the oxygen content in the atmospheric air. If the elemental composition of the fuel is known, the theoretical amount of air L0, kg/kg, is calculated by the formula:
In practice, to ensure the completeness of fuel combustion, an excess amount of air is introduced into the furnace, we find the actual air flow at α = 1.25:
L = aL 0 ,
where L- actual air consumption;
α - coefficient of excess air,
L = 1.25∙17.0 = 21.25 kg/kg.
Specific air volume (n.a.) for combustion of 1 kg of fuel:
where ρ in= 1.293 - air density under normal conditions,
m 3 / kg.
Let's find the amount of combustion products formed during the combustion of 1 kg of fuel:
if the elemental composition of the fuel is known, then the mass composition of flue gases per 1 kg of fuel during its complete combustion can be determined on the basis of the following equations:
where mCO2 , mH2O , m N2 , mO2- mass of the corresponding gases, kg.
Total amount of combustion products:
m p. s = m CO2 + m H2O + m N2 + m O2 ,
m p. s= 2.71 + 2.21 + 16.33 + 1.00 = 22.25 kg/kg.
Checking the received value:
where W f- specific consumption of injector steam during liquid fuel combustion, kg/kg (for gas fuel W f = 0),
Since the fuel is a gas, we neglect the moisture content in the air and do not take into account the amount of water vapor.
Let us find the volume of combustion products under normal conditions formed during the combustion of 1 kg of fuel:
where m i- the mass of the corresponding gas formed during the combustion of 1 kg of fuel;
ρ i- the density of this gas under normal conditions, kg / m 3;
M i is the molar mass of the given gas, kg/kmol;
22.4 - molar volume, m 3 / kmol,
m 3 /kg; 
m 3 /kg;
m 3 /kg; 
m 3 / kg.
The total volume of combustion products (n.a.) at the actual air flow:
V = V CO2 + V H2O + V N2 + V O2 ,
V = 1.38 + 2.75 + 13.06 + 0.70 \u003d 17.89 m 3 / kg.
Density of combustion products (n.a.):
kg / m 3.
Let us find the heat capacity and enthalpy of combustion products of 1 kg of fuel in the temperature range from 100 °C (373 K) to 1500 °C (1773 K) using the data in Table. 2.
Average specific heat capacities of gases with p, kJ/(kg∙K) table 2
| t, °С |
|||||
The enthalpy of flue gases generated during the combustion of 1 kg of fuel:
where with CO2 , with H2O , with N2 , with O2- average specific heat capacities at constant pressure of the corresponding lawn at temperature t, kJ/(kg K);
with t is the average heat capacity of flue gases generated during the combustion of 1 kg of fuel at a temperature t, kJ/(kg K);
at 100 °С: kJ/(kg∙K);
at 200 °С: kJ/(kg∙K);
at 300 °C: kJ/(kg∙K);
at 400 °С: kJ/(kg∙K);
at 500 °С: kJ/(kg∙K);
at 600 °C: kJ/(kg∙K);
at 700 °С: kJ/(kg∙K);
at 800 °С: kJ/(kg∙K);
at 1000 °С: kJ/(kg∙K);
at 1500 °C: kJ/(kg∙K);
The results of the calculations are summarized in Table. 3.
Enthalpy of combustion products Table 3
According to Table. 3 build a dependency graph H t = f ( t ) (Fig. 1) see Attachment .
2.2 Calculation of furnace heat balance, furnace efficiency and fuel consumption
Heat flux taken up by water vapor in the furnace (useful heat load):
where G- the amount of superheated water vapor per unit time, kg/s;
H vp1 and H vp2
We take the temperature of the outgoing flue gases to be 320 °C (593 K). Heat loss by radiation to the environment will be 10%, with 9% of them lost in the radiant chamber, and 1% in the convection chamber. Furnace efficiency η t = 0.95.
Heat losses from chemical underburning, as well as the amount of heat of the incoming fuel and air, are neglected.
Let's determine the efficiency of the furnace:
where uh is the enthalpy of the combustion products at the temperature of the flue gases leaving the furnace, t uh; the temperature of the outgoing flue gases is usually assumed to be 100 - 150 ° C higher than the initial temperature of the raw material at the furnace inlet; q sweat- heat loss by radiation to the environment, % or fraction of Q floor ;
Fuel consumption, kg/s:
kg/s.
2.3 Calculation of the radiant chamber and convection chamber
We set the flue gas temperature at the pass: t P\u003d 750 - 850 ° С, we accept
t P= 800 °C (1073 K). Enthalpy of combustion products at the temperature at the pass
H P= 21171.8 kJ/kg.
Heat flux taken up by water vapor in radiant tubes:
where H n is the enthalpy of combustion products at flue gas temperature at the pass, kJ/kg;
η t - efficiency of the furnace; it is recommended to take it equal to 0.95 - 0.98;
Heat flux taken up by water vapor in convection pipes:
The enthalpy of water vapor at the entrance to the radiant section will be:
kJ/kg.
We accept the value of pressure losses in the convection chamber ∆ P to= 0.1 MPa, then:
P to = P - P to ,
P to= 1.2 - 0.1 = 1.1 MPa.
Temperature of water vapor inlet to the radiant section t to= 294 °C, then the average temperature of the outer surface of the radiant tubes will be:
where Δt- the difference between the temperature of the outer surface of the radiant pipes and the temperature of the water vapor (raw material) heated in the pipes; Δt= 20 - 60 °С;
TO.
Maximum design combustion temperature:
where t o- reduced temperature of the initial mixture of fuel and air; taken equal to the temperature of the air supplied for combustion;
THX.- specific heat capacity of combustion products at temperature t P;
°C.
At tmax = 1772.8 °С and t n \u003d 800 ° C heat density of an absolutely black surface qs for various temperatures of the outer surface of radiant tubes has the following values:
Θ, °С 200 400 600
qs, W/m2 1.50 ∙ 10 5 1.30 ∙ 10 5 0.70 ∙ 10 5
We build an auxiliary chart (Fig. 2) see Attachment, according to which we find the heat density at Θ = 527 °С: qs\u003d 0.95 ∙ 10 5 W / m 2.
We calculate the total heat flux introduced into the furnace:
Preliminary value of the area equivalent to a completely black surface:
m 2.
We accept the degree of masonry screening Ψ = 0.45 and for α = 1.25 we find that
Hs /H l = 0,73.
The value of the equivalent flat surface:
m 2.
We accept a single-row placement of pipes and a step between them:
S = 2d n= 2 ∙ 0.152 = 0.304 m. For these values, the form factor To = 0,87.
The value of the shielded masonry surface:
m 2.
Heating surface of radiant tubes:
m 2.
We choose the BB2 oven, its parameters:
radiation chamber surface, m 2 180
convection chamber surface, m 2 180
working length of the furnace, m 9
radiation chamber width, m 1.2
version b
fuel combustion method flameless
radiation chamber pipe diameter, mm 152×6
convection chamber pipe diameter, mm 114×6
Number of pipes in the radiation chamber:
where d n is the outer diameter of the pipes in the radiation chamber, m;
l floor - the useful length of the radiant pipes, washed by the flow of flue gases, m,
l floor = 9 - 0.42 = 8.2 m,
.
Thermal stress of the surface of radiant tubes:
W / m 2.
Determine the number of convection chamber pipes:
We arrange them in a checkerboard pattern of 3 in one horizontal row. Step between pipes S = 1.7 d h = 0.19 m.
The average temperature difference is determined by the formula:
°C.
Heat transfer coefficient in the convection chamber:
W / (m 2 ∙ K).
The heat stress of the surface of convection pipes is determined by the formula:
W / m 2.
2.4 Hydraulic calculation of the furnace coil
The hydraulic calculation of the furnace coil consists in determining the pressure loss of water vapor in radiant and convection pipes.
where G
ρ to v.p. - the density of water vapor at an average temperature and pressure in the convection chamber, kg / m 3;
d k – internal diameter of convection pipes, m;
z k is the number of flows in the convection chamber,
m/s.
ν k \u003d 3.311 ∙ 10 -6 m 2 / s.
The value of the Reynolds criterion:
m.
Friction pressure loss:
Pa = 14.4 kPa.
Pa = 20.2 kPa.
where Σ ζ to
- number of turns.
Total pressure loss:
2.5 Calculation of water vapor pressure loss in the radiation chamber
Average steam velocity:
where G is the flow rate of water vapor superheated in the furnace, kg/s;
ρ r v.p. - the density of water vapor at an average temperature and pressure in the convection chamber, kg / m 3;
dр – internal diameter of convection pipes, m;
z p is the number of flows in the clnvection chamber,
m/s.
Kinematic viscosity of water vapor at average temperature and pressure in the convection chamber ν p \u003d 8.59 ∙ 10 -6 m 2 / s.
The value of the Reynolds criterion:
Total length of pipes in a straight section:
m.
Hydraulic friction coefficient:
Friction pressure loss:
Pa = 15.1 kPa.
Pressure loss to overcome local resistance:
Pa = 11.3 kPa,
where Σ ζ p\u003d 0.35 - resistance coefficient when turning by 180 ºС,
- number of turns.
Total pressure loss:
The calculations performed showed that the selected furnace will provide the process of superheating water vapor in a given mode.
3. Calculation of the waste heat boiler
Find the average flue gas temperature:
where t 1 - flue gas temperature at the inlet,
t 2 – outlet flue gas temperature, °С;
°C (538 K).
Flue gas mass flow:
where B - fuel consumption, kg / s;
For flue gases, specific enthalpies are determined based on the data in Table. 3 and fig. 1 according to the formula:
Enthalpies of coolants Table 4
Heat flux transmitted by flue gases:
where H 1 and H 2 - enthalpy of flue gases at the temperature of the inlet and outlet of the KU, respectively, formed during the combustion of 1 kg of fuel, kJ/kg;
B - fuel consumption, kg/s;
h 1 and h 2 - specific enthalpies of flue gases, kJ / kg,
Heat flux perceived by water, W:
where η ku - coefficient of heat utilization in CU; η ku = 0.97;
G n - steam capacity, kg/s;
h k vp - enthalpy of saturated water vapor at outlet temperature, kJ/kg;
h n in - feed water enthalpy, kJ/kg,
The amount of water vapor received in the KU is determined by the formula:
kg/s.
Heat flux taken up by water in the heating zone:
where h k in - specific enthalpy of water at the evaporation temperature, kJ / kg;
Heat flux transferred by flue gases to water in the heating zone (useful heat):
where h x is the specific enthalpy of flue gases at temperature t x , from here:
kJ/kg.
The value of the enthalpy of combustion of 1 kg of fuel:
According to fig. 1 flue temperature corresponding to the value H x = 5700.45 kJ/kg:
t x = 270 °С.
Average temperature difference in the heating zone:
°C.
270 flue gases 210 Taking into account the counterflow index:
where To f is the heat transfer coefficient;
m 2.
Average temperature difference in the evaporation zone:
°C.
320 flue gases 270 Taking into account the counterflow index:
187 water vapor 187
Heat exchange surface area in the heating zone:
where To f is the heat transfer coefficient;
m 2.
Total heat exchange surface area:
F = F n + F u ,
F\u003d 22.6 + 80 \u003d 102.6 m 2.
In accordance with GOST 14248-79, we select a standard evaporator with a vapor space with the following characteristics:
casing diameter, mm 1600
number of tube bundles 1
number of pipes in one bundle 362
heat exchange surface, m 2 170
sectional area of one stroke
through pipes, m 2 0.055
4. Heat balance of the air heater
Atmospheric air with temperature t ° in-x enters the apparatus, where it is heated to a temperature t x in-x due to the heat of flue gases.
Air consumption, kg / s is determined based on the required amount of fuel:
where AT- fuel consumption, kg/s;
L- actual air consumption for combustion of 1 kg of fuel, kg/kg,
Flue gases, giving off their heat, are cooled from t dg3 = t dg2 before t dg4 .
=
where H3 and H4- flue gas enthalpies at temperatures t dg3 and t dg4 respectively, kJ/kg,
Heat flux perceived by air, W:
where with in-x- average specific heat capacity of air, kJ/(kg K);
0.97 - air heater efficiency,
Final air temperature ( t x in-x) is determined from the heat balance equation:
TO.
5. Heat balance of KTAN
After the air heater, flue gases enter the contact apparatus with an active nozzle (KTAN), where their temperature decreases from t dg5 = t dg4 up to temperature t dg6= 60 °С.
The flue gas heat is removed by two separate water flows. One stream comes into direct contact with flue gases, and the other exchanges heat with them through the coil wall.
Heat flow given off by flue gases, W:
where H5 and H6- flue gas enthalpies at temperature t dg5 and t dg6 respectively, kJ/kg,
The amount of cooling water (total), kg/s, is determined from the heat balance equation:
where η - KTAN efficiency, η=0.9,
kg/s.
Heat flux perceived by cooling water, W:
where G water- cooling water consumption, kg/s:
with water- specific heat capacity of water, 4.19 kJ/(kg K);
t n water and t to water- water temperature at the inlet and outlet of the KTAN, respectively,
6. Calculation of the efficiency of the heat recovery plant
When determining the value of the efficiency of the synthesized system ( η mu) the traditional approach is used.
The calculation of the efficiency of the heat recovery plant is carried out according to the formula:
7. Exergy assessment of the "furnace - waste heat boiler" system
The exergetic method of analysis of energy technological systems allows the most objective and qualitative assessment of energy losses, which are not detected in any way during a conventional assessment using the first law of thermodynamics. In the case under consideration, the exergy efficiency is used as an evaluation criterion, which is defined as the ratio of the exergy removed to the exergy supplied to the system:
where E sub- fuel exergy, MJ/kg;
E resp.- exergy taken up by the flow of water vapor in the furnace and waste heat boiler.
In the case of gaseous fuel, the supplied exergy is the sum of the exergy of the fuel ( E sub1) and air exergy ( E sub2):
where N n and But- air enthalpies at the furnace inlet temperature and ambient temperature, respectively, kJ/kg;
That- 298 K (25 °С);
∆S- change in air entropy, kJ/(kg K).
In most cases, the value of air exergy can be neglected, that is:
The allotted exergy for the system under consideration is the sum of the exergy taken up by water vapor in the furnace ( E resp1), and the exergy taken up by water vapor in the CH ( E resp2).
For steam flow heated in a furnace:
where G- steam consumption in the furnace, kg/s;
H vp1 and H vp2- enthalpies of water vapor at the inlet and outlet of the furnace, respectively, kJ/kg;
ΔS vp- change in entropy of water vapor, kJ/(kg K).
For the flow of water vapor obtained in the HV:
where G n- steam consumption in CU, kg/s;
h to ch- enthalpy of saturated water vapor at the outlet of the KU, kJ/kg;
h n in- enthalpy of feed water at the inlet to the KU, kJ/kg.
E resp. = E otv1 + E otv2 ,
E resp.\u003d 1965.8 + 296.3 \u003d 2262.1 J / kg.
Conclusion
Having carried out the calculation for the proposed installation (recovery of the heat of the waste gases of the process furnace), we can conclude that for a given fuel composition, furnace productivity in terms of water vapor, and other indicators, the efficiency of the synthesized system is high, thus, the installation is effective; this was also shown by the exergy assessment of the system "furnace - waste heat boiler", however, in terms of energy costs, the installation leaves much to be desired and needs to be improved.
List of used literature
1. Haraz D .And. Ways of using secondary energy resources in chemical industries / D. I. Kharaz, B. I. Psakhis. - M.: Chemistry, 1984. - 224 p.
2. Scoblo A . And. Skoblo A.I., Tregubova I.A., Yu.K., Molokanov. - 2nd ed., revised. and additional – M.: Chemistry, 1982. – 584 p.
3. Pavlov K .F. Examples and tasks in the course of processes and apparatuses of chemical technology: Proc. Manual for universities / K. F. Pavlov, P. G. Romankov, A. A. Noskov; Ed. P. G. Romankova. - 10th ed., revised. and additional - L.: Chemistry, 1987. - 576 p.
Appendix
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 a chimney. Indeed, the more intensely the firewood burns, the more hot flue gases should be, the greater the thrust (carburetor model) should be. 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 cross-sectional area of the chimney. A number of practical conclusions follow from this.
First of all, 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 in the volumetric air flow through the firebox.
Let us explain this with a simple and illustrative 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 discharged 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 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 have to be replaced by a modern system of forced flue gas extraction using an electric fan with adjustable flow and with pre-condensation of moisture from the 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 the very least, if the stove is operating in the long-term burning mode (smoldering firewood), then pipe insulation is absolutely necessary, since at low powers (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 prohibited in 1991 (and even earlier on chimneys of gas stoves).
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 used very widely 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 the rafters (or beams) for the design of a fire opening (cutting) in the roof or ceiling (if necessary); 7 - roof ridge; 8 - soft roofing (roofing material, hydrostekloizol, soft tiles, corrugated cardboard-bitumen sheets, etc.); 9 - metal sheet for roofing and covering the opening (it is allowed to use a flat sheet of aceid - an 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 typical installation diagrams of non-insulated metal pipes are presented. 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 the many misunderstandings of our 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 amount of water vapor is formed, sometimes exceeding the volume of CO2. The presence of moisture in the top
|
Table 36 Heat capacity, kcal/(m3. °C) |
Live, naturally, increases the content of water vapor in the combustion products.
Composition of products complete combustion the main types of fuel in the stoichiometric volume of air are 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 of CO2, which has the highest volumetric heat capacity, there are 3.76 m3 of N2 with a minimum volumetric
|
Average volumetric heat capacities of carbon and hydrogen combustion products in the theoretically required amount of air, kcal/(m3-°C)
|
Heat capacity, and in the products of hydrogen combustion for every 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 the heat capacity of the products of combustion of various types of fuel 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
|
The value of the average heat capacities of the combustion products of fuel and air not diluted with air in the temperature range from 0 to t ° С
|
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
|
Heat capacity of combustion products from 0 to t’mayL of various types of solid fuels with a moisture content of 0 to 40%, in a stoichiometric volume of air
|
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 output of these types of fuel (wood, peat, brown coal, mixed generator, air and blast-furnace gases) is 1600-1700 °C.
Table 40
|
The ratio of the heat capacities of combustion products C 'and air K in the temperature range from 0 to t ° C to the heat capacity of combustion products from 0 to
|
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.