CC BY
0© Parviz S. Khujaev, 2023
UDC 621.182.9
2.1.3 Heat supply, ventilation, air conditioning, gas supply and illumination (engineering science)
THE EFFICIENCY OF LOW-POWER SOLID-FUEL WATER BOILERS
Parviz S. Khujaev
Tajik Technical University named after academician M. S. Osimi, Dushanbe, Republic of Tajikistan
ЭФФЕКТИВНОСТЬ МАЛОМОЩНЫХ ТВЕРДОТОПЛИВНЫХ ВОДОГРЕЙНЫХ КОТЛОВ
П. С. Хужаев
Таджикский технический университет имени академика М. С. Осими, Душанбе, Республика Таджикистан
Abstract. Burning fuel in large quantities when using hot water boilers leads to losses, which reduces the energy conversion efficiency of the boiler. Therefore, there were mainly small energy-saving stoves designed to burn fuel. The author proposed to use a small boiler with a firebox with a vertical grate. A small boiler with an adjustable grate water heater with a capacity of 50 kW was installed in the boarding school building. Combustion of any fuel in such a furnace had following results: a) the productivity of the boiler unit had been increased to 79-81 %; b) heat loss reduced by 2 times and harmful emissions into the atmosphere in 1.5 once. The costs of providing thermal energy compared to other operating boiler houses amounted to 14 400 somoni (1 200 dollars). This is 18 % less than the costs of the previous heating season without using the proposed boiler. The efficiency of heating processes in the boiler was assessed during one heating season.
Key words: hot water boiler, furnace, vertical grate, thermodynamic analysis, energy efficiency, heat loss with flue gases, fuel combustion temperature
Аннотация. Сжигание топлива в больших количествах при использовании водогрейных котлов приводит к определенным потерям, что снижает КПД котла. Поэтому для сжигания топлива были разработаны малогабаритные энергосберегающие печи. Автор предложил использовать малогабаритный котел с топкой с вертикальной колосниковой решеткой. Малогабаритный котел с водонагревателем с регулируемой колосниковой решеткой мощностью 50 кВт был установлен в здании школы-интерната. Сжигание любого топлива в такой топке привело к следующим результатам: а) производительность котлоагрегата увеличилась до 79-81 %; б) потери тепла сократились в2 раза, а вредные выбросы в атмосферу - в 1.5 раза. Затраты на обеспечение тепловой энергией по сравнению с другими действующими котельными составили 14 400 сомони (1 200 долл.). Это на 18 % меньше затрат предыдущего отопительного сезона без использования предлагаемого котла. Эффективность тепловых процессов в котле оценивалась в течение одного отопительного сезона.
Ключевые слова: водогрейный котел, топка, вертикальная колосниковая решетка, термодинамический анализ, энергоэффективность, потери тепла с дымовыми газами, температура сгорания топлива
DOI 10.31660/2782-232X-2023-4-48-54 Архитектура, строительство, транспорт
2023. № 4 (106). С. 48-54
For citation: Khujaev, P. S. (2023). The efficiency of low-power solid-fuel water boilers. Architecture, Construction, Transport, (4(106)), pp. 48-54. (In English). DOI 10.31660/2782-232X-2023-4-48-54.
Для цитирования: Хужаев, П. С. Эффективность маломощных твердотопливных водогрейных котлов / П. С. Хужаев. - 001 10.31660/2782-232Х-2023-4-48-54. - Текст : непосредственный // Архитектура, строительство, транспорт. - 2023. - № 4 (106). - С. 48-54.
Introduction
In Tajikistan, one of the main types of accessible and economically viable fuel is coal and lignite. At the same time, coal has disadvantages, such as high heat loss due to mechanical and chemical underburning, and a negative impact on the environment due to dangerous emissions of carbon dioxide and combustion products. In this regard, low-power solid fuel hot water boilers are traditionally used for heating and hot water supply [1].
The main types of fuel for such stoves are wood, coal and manure - local fuel, which is waste from livestock farming. Solid fuel obtained from different deposits differ from each other in their properties and combustion products.
The main values of the thermophysical properties of coal from some deposits in Tajikistan are presented in Table 1.
The efficiency of thermal processes in the boiler was evaluated using the first and second laws of thermodynamics. Based on the energy balance,
the boiler efficiency was assessed [2-4]. The work is based on well-known research in the field of heat generating plants by K. F. Roddatis, A. N. Poltaretsky, V. M. Fokin, E. G. Volkovyskiy, N. B. Lieberman, Yu. L. Gusev. It is also necessary to note the works of A. V. Vikhrov and L. K. Ramzin, which address the issues of the organization of solid fuel combustion and the design features of boilers with furnaces of layered construction.
Since the 90s of the twentieth century, research has been carried out on the conditions for the efficient combustion of fuels of various compositions without the formation of harmful emissions into the atmosphere. Research is mainly aimed at identifying methods, mechanisms for controlling and organizing fuel combustion processes with minimal emissions of harmful substances into the atmosphere. The most effective methods for calculating thermal conditions in a limited volume are described in the works [5-8]. Research on this topic was conducted by [9-11].
Thermophysical properties of coal of the main deposits in Tajikistan Термофизические свойства угля основных месторождений Таджикистана
Table 1 Таблица 1
Name of the deposits Heat of combustion, kJ/kg Humidity, % Ash content, % Yield of volatile substances, %
Hakimi 27 017-32 573 5.2-17.4 4.8-19.7 26.8-46.9
Fan-Yagnob 32 260 0.35-1.88 3.9-24.5 23.1-36.9
Sayat 30 919 4.9 32.3 32.43
Ziddi 28 085-32 175 3.4-10.2 12.6-33.7 4.9-25.8
Mienadu 35 026 1.1 11.1-30 4.9-25.8
Ravnou 31 694 0.3-9.6 6.3-34.8 29.3-38.3
Nazar-Aylock 35 145 0.78-4.66 1.2-4.2 3.0-5.0
Shurab 27 964 13.1 12.5 34.8
However, in existing publications, practically no attention is paid to optimizing the operating modes of low-power solid fuel water boilers when operating on coal with high ash content. In existing small boilers there is no solid fuel discharge, the firebox has only one burner, and the fireplace grate is fixed. My colleagues and I have also devoted our research to exploring this topic [12-14]. In the design of a small boiler proposed by the author, all types of fuel are burned; the grate inside the firebox is movable in the vertical direction. A distinctive feature of the circuit is the presence of a small water heater boiler. The volume of the firebox has increased for burning other solid fuels, taking into account their characteristics.
The purpose of the study is to develop the design of a hot water boiler for efficient combustion of fuels of various compositions. To develop the design of a water-heating boiler, a mathematical description of the combustion processes of fuel when the volume of the combustion space changes was carried out. A design of a water-heating boiler with a movable grate was developed, and experimental studies of the combustion of high-ash fuel on a movable grate were carried out.
The thickness of the radiating layer is calculated according to the formula:
s = 3.6 • V /Fst,
where Vt is the volume of the furnace chamber, m3, F, - the surface area of the furnace walls, m2.
st '
The issues related to the energy and heat release analysis of the efficient combustion of solid fuel can be quite broad and complex. However, here are several factors that should be considered when analyzing the efficiency of low-power solid fuel hot water boilers:
1. A method used to conduct an energy analysis of the solid fuel combustion.
2. Factors affecting the efficiency of solid fuel combustion.
3. Parameters and indicators used to analyze heat release in solid fuel combustion.
4. Existing technologies for improving the efficiency of solid fuel combustion.
5. Environmental aspects that should be considered in the analysis of solid fuel combustion.
6. Opportunities to optimize the solid fuel combustion process to achieve maximum energy efficiency and minimum exhaust emissions.
Exergetic analysis is one of the tools for evaluating the effectiveness of thermodynamic processes. It allows us to determine the maximum possible use of energy that can be extracted from the system to perform useful work.
When exergetic analysis is applied to boilers, it helps to determine the level of energy losses associated with processes such as heat exchange, diffusion, friction and others. This makes it possible to identify problem areas that can be optimized to increase boiler efficiency.
Object and methods of research
The object of the study is a hot water boiler unit with the vertical grate of furnace.
The thermodynamic efficiency of a system at equilibrium includes:
1. Efficiency (reduces the energy conversion efficiency) is the ratio of useful energy used to energy expended. In the case of a reversible transition, efficiency can be defined as the ratio of work obtained to heat expended.
2. Thermal efficiency is the ratio of heat generated to heat consumed. In a reversible process, the thermal efficiency will be maximum and equal to unity.
3. Exergy efficiency is the ratio of released exergy (useful work) to stored exergy (expended heat). In a reversible process, the exergy efficiency will be equal to unity.
In an adiabatic process, the efficiency will be high since the system does not lose or release thermal energy.
Let us consider a method for studying technological and exergy processes based on the second law of thermodynamics. The action of these processes occurs under conditions of interaction of the system with an equilibrium environment. Environmental parameters ensure the constancy
Fig. 1. Diagram of a hot-water sectional solid fuel boiler: 1 - firebox; 2 - movable grate; 3 - lifting mechanism;
4 - the beam-receiving surface (additional drum); 5 - the transition gas pipe; 6 - the lower gas distribution chamber outlet; 7 - the main drum of the boiler; 8 - the central exhaust pipe; 9 - upper gas distribution chamber; 10 - exhaust pipes
with descending flows; 11 - exhaust pipes with ascending flows; 12 - thermometer; 13 - chimney with gate; 14 - cold water inlet pipe; 15 - heated water outlet pipe; 16 - transition pipe; 17 - air outlet pipe; 18 - ash pan; 19 - thermometer
Рис. 1. Схема водогрейного секционного твердотопливного котла: 1 - топка; 2 - подвижная колосниковая
решетка; 3 - подъемный механизм; 4 - лучевоспринимающая поверхность (дополнительный барабан);
5 - переходная газовая труба; 6 - нижняя газораспределительная камера отводная; 7 - основной барабан котла; 8 - центральная газоотводная труба; 9 - верхняя газораспределительная камера; 10 - газоотводные
трубы с нисходящими потоками; 11 - газоотводные трубы с восходящими потоками; 12 - термометр;
13 - дымоход с шибером; 14 - патрубок входа холодной воды; 15 - патрубок выхода нагретой воды;
16 - переходной патрубок; 17 - патрубок выпуска воздуха; 18 - зольник; 19 - термометр
of the thermodynamic system, and energy is where Eentis exergy supplied to the object with flows
converted into work. There are several types of of matter and energy;
work. To improve the efficiency of the boiler, the Eex - exergy leaving the installation;
concept of chemical filling can be used [2-5, 10-14]. EDi - exergy losses.
If calculate the exergy balance, you can take into The article discusses the equipment of a furnace
account the excess lost in the process of switching with a variable volume of combustion space. Coal
between components: from the Ziddi deposit was used to produce flue
gases. Their temperature was 300 K. In the mixing
+ chamber, the combustion products were diluted
with air (To = 293 K, moisture content d = 0.01 kg/kg, p = 9.8 • 104 Pa. The air consumption for complete combustion of 1 kg of fuel is 46.7m3/kg [4].
The exergy balance for the furnace is determined by the formula:
Et =Ern +A Etheon„ +AE,
t cp the env. '
where Et is exergy of the initial fuel Et = 5 383 kcal/kg = 22 538 kJ/kg;
Ecp - exergy of combustion products in the furnace
at adiabatic combustion temperature;
AE. - exergy of the heat flow into the
the env. -1 J
environment;
AE - Total losses in the furnace.
According to calculations, the energy conversion efficiency of the furnace can be about 89 %. The operation of the furnace with a variable volume of combustion space is 15-20 % more efficient than standard furnace.
Results
Therefore, the exergy efficiency of the boiler nex characterizes the share of usefully used exergy:
nex =-
^exit ^redu ^entrance ^redu
where E t, E are spent and used exergy,
cost ser 1 ~JJ
respectively; Eredu - common transit exergy, that is, exergy that passes from the entrance to the equipment to its exit without participation in energy exchange processes. For the waste heat boiler in this case, the transit exergy includes the exergy of the feed water flows E'va and the air E'airas well as the physical exergy of the exhaust gas flow of the soot production. The coal from the Ziddi deposit has the following characteristics of its combustion, m3/kg: VQ0= 5.51; VCOi = 0.915; VN= 4.2; ^0= 0.615; Vcp = 5.73. For coal from the Ziddi deposit (Q"p= 22500kJ/kg) the exergy of coal combustion products was calculated (table 2, 3).
Let us determine the exergy of flue gases in the mixture obtained by diluting the combustion products of coal with air in the amount of 46.7 m3/kg; p = po = 9.8 • 104 Pa, T0 = 293 K, W0 = 0.01 kg/kg up to the temperature of the mixture t = 573 K.
1 sm
There were firebox characteristics using in the study:
• hearth surface was 0.2826 m2;
• side surface - 0.7536 m2;
• ceiling surface - 0.2703 m2;
• the inner surface of the central exhaust pipe -0.314 m2;
Table 2 Таблица 2
Exergy of the coal from the Ziddi deposit Эксергия угля Зиддинского месторождения
1/0 VCp, m3/kg Mole fractions Moisture content Xzc/ kg/kg Chemical effect of the combustion process e0 p, KJ/kg
N2 CO2 H20
6.6C3 C.7B9 C.17SC C.C9B3 0.0162 1046
Table 3 Таблица 3
Thermodynamic parameters of flue gas components Термодинамические параметры компонентов дымовых газов
Substances V, m3/kg t N, kmol/m3 h, kJ/kmol h0, kJ/kmol Ah, kJ/ kmol NAh, kmol/ ' kg t S, kJ/(kmolK)
CO2 C.916 C.C412 12 519 7б1 11 751 47B 3C.43
N2 S7.2C 2.SS1 B 7B9 SB1 B211 2C 949 21.б4
O2 1S.1B C.677 9 1CC S9C BS1C S 7б9 22.2б
H2O 1.7S C.C71 1C 351 б73 9б72 B1B 25.5б
Total 7B.64 3.S11 2B C2C
• the surface of the ceiling of the flue pipe -1.2265 m2;
• furnace volume - 1.228525 • 10-1 m3.
The mass flow rate of combustion products during the combustion of 1 kg of solid fuel is determined by the dependence:
A"
Gg = 1--+ 7.306 цVй,
for V [kg] fuel:
G = BG0=B
100
1- —+ 1.306-ccV° 100
= f{B,oc,V°)
or t(x) = t0+exp
knd.
g.tr.h
cf(B,v,V0)'
In the process of energy and exergy analysis of the efficiency of operation of low-power solid fuel hot water boilers by installing a movable grate, it was found that the variable volume of the furnace space, helps us to reduce fuel consumption, and also contributes to the afterburning of volatile gases and unburned pulverized fractions of coal.
The calculation showed that the efficiency of the furnace was 89 % and fuel consumption decreased by 15-20 %. Heat loss from external gases amounted to 8.8 %; heat losses from the use of mechanical fuel amounted to 17.4 %. The productivity of the boiler unit has been increased to 79-81 %. On the issue of emission of harmful substances into the atmosphere, a boiler model with removable grates and combustion technology is considered in the Set of rules 89.13330.20121 .
Conclusion
Based on the research carried out, an urgent scientific problem has been solved - a low-power solid fuel boiler with a movable furnace and the possibility of regulating volumetric thermal voltage has been developed for use in the water heating system of detached buildings to ensure the efficiency of solid fuel combustion. The proposed boiler design ensured a minimum airflow rate for complete combustion of solid fuel in the boiler furnace and a maximum direct heat transfer coefficient in the combustion chamber. The study showed a number of findings:
1. The maximum efficiency of the proposed furnace when burning various solid fuel (coal) is established by changing the furnace space of the boiler using a movable grate, which allows us to adjust the volume of the furnace space.
2. Reducing and increasing the furnace space of a small water heater helps maintain the required boiler temperature, which leads to a reduction in the content of nitrogen oxides by 20-25 %.
3. In the course of economic research and implementation of the developed project, a small boiler with an adjustable grate water heater with a capacity of 50 kW was installed in the boarding school building. Over the period of operation, one season, compared to other existing boiler houses, cost 14 400 somoni (1.200 US dollars), which is 18 % less than the initial costs of previous years for the provision of thermal energy.
Based on the results of the study, a comprehensive automated control system for boiler operating modes will be created, providing regulation of the volume of the combustion space, air supply and removal of combustion products.
References
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Information about the author
Parviz S. Khujaev, Cand. Sc. in Engineering, Associate Professor, Head of the Department of Water Supply Systems, Heat and Gas Supply and Ventilation, Tajik Technical University named after Academician M. S. Osimi, e-mail: pkhujaev@gmail.com
Сведения об авторе
Хужаев Парвиз Саидгуфронович, канд. техн. наук, доцент, заведующий кафедрой систем водоснабжения, теплогазоснабжения и вентиляции, Таджикский технический университет имени академика М. C. Оси-ми, e-mail: pkhujaev@gmail
Получена 04 октября 2023 г., одобрена 23 октября 2023 г., принята к публикации 15 декабря 2023 г. Received 04 October 2023, Approved 23 October 2023, Accepted for publication 15 December 2023
