Design of Low Quality Heat Recovery and Utilization System for Fixed Discharge of Coal-fired Boilers

In order to optimize the energy structure, increase the proportion of clean heat sources, and solve the problem of high energy consumption for heating, a waste heat recovery scheme is planned to be designed, which mainly recovers the waste heat of the 700MW boilers in the thermal power plant, which can be used to heat the condensed water of the steam turbine in summer., It can be used to heat the circulating water in the heating network in winter to achieve the goal of recovering heat and reducing energy consumption. In the design process, a lithium bromide absorption heat pump is used as the main system for low-quality heat recovery, and steam is used as a driving heat source to release heat in the generator to heat a dilute solution of lithium bromide to generate refrigerant vapor, thereby achieving high efficiency, energy saving and economic benefits. the goal of. At the same time, two booster pumps (one for transport and one for standby) are added to the condensate pipeline at the inlet of the low-low temperature economizer system to achieve the purpose of boosting the condensate. This design plan adheres to the concept of energy saving and environmental protection, and proposes a technical transformation route for low-quality heat recovery and utilization projects to achieve the goals of reducing production costs and increasing corporate benefits.


Purpose and significance
When fossil-fuel power station is generating electricity, it has caused a lot of resource waste, especially a large amount of low-temperature waste heat is discharged into the atmosphere, which has caused a great waste of resources and some damage to the environment.While fossil-fuel power station in China supply more than 60% of electricity, they also consume a large amount of coal and industrial water.The content of SO2, NOX and CO2 in the flue gas emitted by power plants accounts for 45%, 50% and 48% of the total emissions in China [1] .In order to respond to the national call for energy conservation and emission reduction and reduce the emission of flue gas from power plants, it is urgent to adopt new energy conservation technologies.Among them, the waste heat energy conservation technology of power plant boilers will directly improve the comprehensive performance of fossil-fuel power station, thus playing a positive role in the national energy conservation and emission reduction work.
In order to reduce the emissions of working fluids and waste of heat during normal operation of the boiler, this design utilizes a low-quality heat recovery device to comprehensively recycle and utilize the boiler wastewater (fixed or continuous discharge wastewater), soot blowing drainage, electric dust tracing drainage, fuel oil sweeping drainage, and heater drainage discharged from the boiler, achieving the goals of energy conservation, water conservation, environmental protection, and improving the corporate image.

Research content
The project mainly revolves around the waste heat recovery system for boiler fixed discharge wastewater.The main research plan is to consider introducing the boiler's fixed discharge drainage separately into a low-quality heat recovery device for recycling and utilization.The recovered heat is used for heating the heating network in winter, and for heating the condensate during the non heating season.The condensate is sent to the inlet of the low-temperature economizer pump, thereby improving the system's economy.The recovered water resources have good water quality and low temperature, and are directly fed into the chemical water system.After treatment, they are used for system desalination water replenishment [2][3][4] .

Project Overview
The project is a 700MW generator unit with a total evaporation of 2200t/h.After the waste water discharged from the boiler is continuously expanded, the generated secondary exhaust steam enters the deaerator, and the boiler waste water is discharged from the drain outlet to the constant discharge expansion tank for secondary flash evaporation.The boiler is scheduled to discharge approximately once every 24 hours, with a discharge rate of around 0.5% [5,6] .

Figure 1. Process Flow Diagram
As shown in Figure 1 above.The project directly recovers the exhausted steam discharged from the fixed discharge expansion vessel through the lithium bromide exhaust steam heat pump.The exhausted steam does not need to undergo secondary conversion and can be directly used as a residual heat source of the heat pump to be fed into the heat pump evaporator.The recovered waste heat can be used for preheating some boiler condensate, as well as for hot water heating in the heating network.

System Configuration
The fixed exhaust steam volume of this project is 2-4t/h, and a certain margin is considered to achieve full recovery of exhaust steam.The exhaust steam volume can be designed according to 5t/h.The waste heat parameters are shown in Table 1.The system is configured according to the exhaust steam volume of 5t/h, with a rated recyclable exhaust heat of 3.5MW.Calculated based on 350 days throughout the year, it is expected to recover and utilize 10.6% of the exhaust steam heat × 104GJ/year.

Calculation of Heat Load for System Heat Pump Design
The principle diagram of an absorption heat pump using exhaust steam as a low-temperature heat source is shown in Figure 2. The intermediate pressure cylinder of the steam turbine shown in Figure 2 is the steam source for the heat pump driving steam and the heating network heater, and is equipped with a pre condenser [7][8] .The design heat load calculation of heat pumps should be carried out according to the following formula:

EPATS-2023 Journal of Physics: Conference Series 2636 (2023) 012037
The known circulating water flow rate is 183m 3 /h, the inlet temperature of the heat pump condensate is 35 ℃, and the outlet temperature of the heat pump condensate is 70 ℃.Therefore, the design heat load of the heat pump is 7449.37kW.

Hydraulic Calculation of Booster Pump
In order to reduce investment and operating costs reasonably, the optimal water supply speed should be selected within an acceptable range according to the regulations.According to the regulations, the average flow rate of the centrifugal pump pipe section for industrial water is 1.5-2.5m/s,and the average flow rate of the pipe section is 2m/s.Based on the assumed average flow rate, the inner diameter of the pipe can be initially selected according to the formula: The volume flow rate of condensate is known to be 183m3/h, with a density of 994kg/m 3 .The mass flow rate can be calculated based on the formula.
Therefore, the inner diameter of the pipe can be calculated: So you can choose DN200 pipes.The average speed is shown in the formula:

Calculation of pipeline pressure loss (1) Friction pressure loss of straight pipes
According to the standard GB 50316-2008 "Design Code for Industrial Metal Pipelines", the calculation formula for frictional pressure loss of circular straight pipes [9] .As shown in the formula.
The friction resistance coefficient, Reynolds number and equivalent diameter of the fluid are calculated according to the formula: Due to the inner diameter of the pipe being 0.2m, which means the effective cross-sectional area of the pipe section is 0.0314m 2 and the wet circumference is 0.628m, the equivalent diameter is 0.2m.Because the dynamic viscosity coefficient of water is 0.4688kg/(s • m) at 60 ℃, the Reynolds number can be calculated as 686.98, and the fluid friction coefficient can be calculated as 0.094.
By taking the known fluid friction coefficient of 0.094, density of 994kg/m 3 , flow rate of 1.62m/s, gravity acceleration of 9.8m/s 2 , pipeline length of 150m, and inner diameter of 0.2m, the friction pressure loss of the straight pipe can be calculated as 0.057921MPa, which is 57921Pa.
(2) Local frictional pressure loss The local pressure loss mainly considers the pressure loss of valves and fittings, and the local resistance coefficients of each component are shown in Table 2 [10] .The calculation of local frictional pressure loss is shown in the formula:

Data Collection and Calculation on Site
The parameter values of the exhaust volume of the drainage tank in the system are divided into two situations, namely, the condition without soot blowing is shown in Table 3, and the condition with soot blowing is shown in Table 4.According to the on-site data collection of a certain factory, it can be determined that the exhaust volume of the deaerator is 1.5t/h, and the total water collection is 8.2t/h.

Conclusion
This design scheme adheres to the management concept of energy conservation and environmental protection, and proposes a technical transformation route for low-grade heat recovery and utilization projects to achieve the goal of reducing production costs and increasing enterprise benefits.

Figure 2 .
Figure 2. Principle Thermodynamic System Diagram of an Absorption Heat Pump Using Exhaust Steam as a Low-temperature Heat Source

Table 2 . 4 .
Local resistance coefficient of pipe fittings ζ.Analysis of energy-saving benefits

Table 1 .
Parameter table of waste heat system.

Table 4 .
With soot blowing conditions.