Boiler evaporation and boiler thermal efficiency
1 ton/hour ≈60× 104 kcal/hour
About 0.7 milliwatts
2. Calculation of thermal efficiency of circulating fluidized bed boiler
1 overview
Hebei Thermal Power Co., Ltd. recently put into operation four circulating fluidized bed boilers, the models of which are DG 4 10/9.438+0? Its main parameters are evaporation 4 10t/h, main steam pressure 9.8 1MPa, main steam temperature 540℃, feed water temperature 225℃, drum pressure1/0.08mpa, bed temperature 896℃ and coal feed rate 46.93t/h. Type 9 circulating fluidized bed boiler has a width of 137 16mm and a depth of 6705mm；; 4 coal feeders on the front wall, each with an output of 36 t/h, and 2 air-cooled combined slag coolers on the left and right; The ignition method is to use the air duct igniter under the bed. There are two air duct igniters under the bed, with the output of 1650kg/h, and four oil guns on the bed, with the output of 500kg/h. The boiler structure is shown on the right.
Since 165438+20021October 30, the boiler operation level has been continuously improved through continuous exploration and summing up experience and lessons, and now it has successfully passed the commissioning period and entered the trial production operation stage. Let's briefly discuss DG 4 10/9.438+0 first. Calculation of thermal efficiency of type 9 circulating fluidized bed boiler.
2. Proposing and analyzing the problem
2. 1 In order to study the thermal efficiency calculation of circulating fluidized bed boiler, we must first understand the difference between circulating fluidized bed boiler and pulverized coal boiler. Compared with pulverized coal boilers, circulating fluidized bed boilers have the following main differences:
(1) The difference between combustion and heat transfer mechanism: Circulating fluidized bed combustion has the characteristics of low temperature and intensified combustion, and its basic principle is that bed materials (below 8mm) burn in fluidized state. Coarse particles burn in the dense phase region of the bed, while fine particles burn in the dilute phase region. Fine particles carried out of the furnace by flue gas are collected by cyclone separator and returned to the bed for circular combustion through "J" valve. Because of the different combustion mechanism, the heat transfer process is also different, which mainly includes three processes: gas convection heat transfer, radiation heat transfer and particle convection heat transfer. Among them, because the gas is mixed with solid particles, the specific constant volume heat capacity of suspended solids is bound to be greater than that of pure gas, so the convective heat transfer of particles accounts for a large proportion.
(2) Different design structure: According to the different combustion and heat transfer mechanisms, compared with pulverized coal boilers, the circulating fluidized bed boilers are equipped with steam-cooled cyclone separators, "J" valve feeders, air-cooled combined slag coolers and other supporting equipment. Among them, the main functions of steam-cooled cyclone separator and "J" valve feeder are to form the material circulation inside the boiler; The main function of the combined air-water cooling slag cooler is to discharge the materials in the lower layer of the furnace through the slag cooler, so as to maintain a reasonable bed pressure difference and ensure the normal fluidization of the bed materials.
(3) The desulfurization process is different: the desulfurizer (limestone) is directly sent to the hearth of the circulating fluidized bed boiler, and the calcined calcium oxide reacts with the sulfur dioxide gas generated by combustion, and the generated calcium sulfate is discharged from the hearth through the slag cooler, thus achieving the purpose of desulfurization. Because the normal bed temperature of the boiler is just the best temperature range for desulfurization (850℃~ 900℃), and because of the repeated circulation of materials in the furnace, the residence time of desulfurizer in the furnace is prolonged, so that the desulfurization efficiency can reach about 90%.
2.2 take our company DG4 10/9.8 1 as an example. Taking a circulating fluidized bed boiler as an example, the calculation of its thermal efficiency is discussed.
In the steady state, the thermal balance equation of the boiler relative to 1Kg coal is as follows:
QR = q1+Q2+Q3+Q4+Q5+Q6 (KJ/kg), and the corresponding percentage heat balance equation is:
100% = q 1+Q2+Q3+Q4+q5+q6(%)
In ...
(1) Qr is the total heat of 1Kg coal input into the boiler, KJ/Kg.
Qr= Qar+hrm+hrs+Qwl
Among them, the low calorific value of Qar coal is kj/kg; It is the main source of boiler heat input.
Physical sensible heat of hrm coal combustion, kj/kg; The temperature of coal combustion is generally lower than 30℃, and this heat is relatively small.
Physical sensible heat of hrs relative to 1Kg coal-fired limestone, kj/kg; This item has relatively little heat.
Qwl uses 1Kg coal-fired air heated outside the boiler, kj/kg; If the heater at the inlet of primary and secondary air is not put into use, this part of heat can be excluded.
(2) Q 1 is the effective heat utilization of the boiler, kj/kg; When calculating the anti-equilibrium thermal efficiency, it is obtained by using other heat losses.
(3) Q4 is the heat loss of mechanical incomplete combustion, KJ/Kg.
Q4 = Qcc(MHz chz+mfh cfh+mdh CDH)/m coal
The calorific value of residual carbon in Qcc ash is KJ/Kg.
Mhz, Mfh and Mdh are the slag discharge, fly ash and bottom ash per hour of the boiler slag cooler, respectively, t/h.
Chz, Cfh and Cdh account for% of residual carbon content in slag, fly ash and bottom ash per hour of boiler slag cooler, respectively.
Coal consumption per hour of coal-fired boiler, t/h
q4= 100Q4/Qr(%)
(4) Q2 is the heat loss of exhaust gas, KJ/Kg.
Q2 =(Hpy-Hlk)( 1-Q4/ 100)
In the formula, the exhaust enthalpy of Hpy is calculated from exhaust temperature θpy (℃), excess air coefficient α py (α py = 21.0/(21.0-o2py)) and specific heat capacity of exhaust volume Cpy (KJ/(Nm3℃)), which is KJ/Kg.
The enthalpy of Hlk cold air entering the furnace is calculated by the excess air coefficient αpy at the exhaust port, the specific heat capacity of cold air Clk (KJ/(Nm3℃), the cold air temperature θlk (℃) and the theoretical air volume VO (VO = 0.0889 (CAR+0.375SAR)+0.265HAR-0.0333OAR, Nm3/kg).
q2= 100Q2/Qr(%)
(5) Q3 is the heat loss of chemical incomplete combustion, KJ/Kg.
Q3 = 236(Car+0.375 sar)(Mco/28)/(MSO 2/64+Mnox/46)( 1-Q4/ 100)
Among them, Mco, Mso2 and Mnox are the mass of co, so2 and nox in flue gas, mg/ Nm3 respectively.
q3= 100Q3/Qr(%)
(6) Q5 is the heat loss of the boiler, KJ/Kg.
q5=(0.28*4 10.0)/H
Where the evaporation of H boiler in actual operation, t/h
(7) Q6 is the physical heat loss of boiler ash, KJ/Kg.
q6 =(HhzMhz * 100/( 100-Chz)+HfhMfh * 100/( 100-Cfh)+hdh mdh * 100/( 100-Cdh))/Mcoal
Where Hhz, Hfh and Hdh are the enthalpy values of slag discharge, fly ash and bottom ash of boiler slag cooler, KJ/Kg respectively, which are calculated from their corresponding average specific heat capacity and temperature.
q6= 100Q6/Qr(%)
(8) η is the anti-equilibrium thermal efficiency of the boiler,%.
η= 100-(q2+q3+q4+q5+q6)
3 Conclusion
Combined with the actual operation data, the calculated value of boiler thermal efficiency is compared with the design data provided by the manufacturer as follows: (rated working condition)
serial number
project
sign
unit
Real data
Design data
1
waste-heat rejection
q2
%
5. 19
5. 1
2
Heat loss of chemical incomplete combustion
q3
%
0.43
0. 1
three
Heat loss of mechanical incomplete combustion
q4
%
3.30
2.5
four
Heat dissipation loss
q5
%
0.28
0. 14
five
Physical heat loss of ash
q6
%
0.77
0.70
six
Anti-equilibrium thermal efficiency
η
%
90.03
9 1.46
According to the difference between the actual operation data and the design data, in order to reduce various heat loss indexes and improve the thermal efficiency of the boiler, we have made improvements in the following aspects:
(1) Try to lower the exhaust temperature. When the tail heating surface has been determined, the number of soot blowing on the tail heating surface should be appropriately increased as needed. By soot blowing, the ash degree of the tail heating area is reduced, and the phenomenon of local ash blocking is avoided, so that the heat transfer temperature and pressure of the tail heating surface are improved, the exhaust temperature is reduced, and the exhaust heat loss is reduced.
(2) According to the combustion mechanism of circulating fluidized bed boiler, it is necessary to ensure the full fluidization of materials in the bed. There are two main aspects: first, it is necessary to ensure a stable fluctuation range of bed pressure, and put in a corresponding number of slag coolers in time according to the changes of coal quality and limestone quantity to avoid the bed pressure from rising too high; At the same time, when the bed pressure drops to a lower level, the slag cooler should be stopped in time for purging. Secondly, it is necessary to ensure that the primary fluidization air volume is greater than the minimum fluidization air volume and increase it appropriately according to the bed temperature. Only by ensuring the full fluidization of the materials in the bed can we avoid some undesirable phenomena such as local coking in the bed, large deviation of bed temperature and local fluidization dead zone, and make the coal in the furnace fully burn, thus reducing the mass content of residual carbon Chz in the slag discharge of the boiler slag cooler and reducing the heat loss of mechanical incomplete combustion.
(3) Pay enough attention to the reliable operation of slag cooler. On the one hand, it is necessary to ensure the reliable slag discharge of the slag cooler and control the hearth pressure; On the other hand, it is necessary to control the operating parameters of slag cooler and reduce the slag temperature to reduce the physical heat loss of slag.
(4) Further adjust the proportion of primary air and secondary air in the furnace. The primary fluidization air ensures that the materials are fully fluidized, and at the same time, it also ensures that there is a certain combustion share in the dense phase area of the furnace, so that the actual excess air coefficient in the dense phase area is close to 1, and it is in an anoxic combustion state. The secondary air enters from the boundary between the dense phase zone and the dilute phase zone of the furnace, and the total air volume required for combustion is controlled according to O2% to ensure the full combustion of fine particles in the dilute phase zone. In addition, the combined action of primary air and secondary air ensures the circulation rate of materials in the furnace, improves the probability of fine particle reburning, reduces the carbon content of fly ash Cfh, and further reduces the heat loss of mechanical incomplete combustion.
(5) Strengthen the improvement of the external thermal insulation material of the boiler, timely repair the defects found, and reduce the heat loss of the boiler.
References:
[1] Cen Kefa, Ni Mingjiang, etc. Theoretical design and operation of circulating fluidized bed boiler. Beijing: China Electric Power Press, 1997.
[2] Liu Dechang editor. Industrial application of fluidized bed combustion technology. Beijing: China Electric Power Press, 1998.