Preamble

Optimization Study on Heating System Stability of Denitrification Fluidized Bed

Wei Wang¹, Zhiyong Liu¹,², Chao Wen¹, Xianwei Xu¹,², Guosheng Bao¹
(1. China National Nuclear Corporation 404 Co., Ltd., Lanzhou, Gansu 732850, China)

Project Type: Gansu Provincial Science and Technology Program Project
Project Number: 23CXKA0013
Corresponding Author: Wei Wang (1998–), male, from Wuhan, Hubei, holds a bachelor's degree and works as an assistant engineer, specializing in electrical system operation research. E-mail: 1299481052@qq.com

Abstract

This paper addresses the operational instability of denitrification fluidized bed heating systems through improvements in external heating methods, upgrades to internal heating rod materials, and optimization of control systems. By replacing traditional radiant heating furnaces with conductive heating furnaces, thermal efficiency increased by 15% while furnace wall temperature decreased by 80°C. Internal heating rods made of Inconel718/GH4169 alloy exhibited significantly enhanced high-temperature and corrosion resistance. A PID-based temperature control system with power compensation programming achieved precise temperature control within ±2°C. Experimental results demonstrate a 20% reduction in energy consumption and a 90% decrease in heating rod failure rates, providing an effective solution for stable fluidized bed operation in high-temperature acidic environments.

Keywords: denitrification fluidized bed; heating system; conductive heating; Inconel718 alloy; PID control

1 Introduction

The denitrification fluidized bed primarily consists of the equipment body, electric heating control system, insulation layer, and external heating furnace support frame, with main applications in nuclear chemical facilities. The equipment body comprises a fluidized bed heating chamber, atomizing nozzle, bed body, backflush gas chamber, and backflush components. This equipment is primarily used for powder product manufacturing, with final collection at the discharge port. To maintain internal bed temperature, reduce heat dissipation, and lower energy consumption, the equipment exterior is wrapped with an insulation layer, as shown in Figure 1 [FIGURE:1].

The heating system comprises an external heating furnace (upper, middle, and lower electric furnaces in Figure 1) and internal heating rods (red components in Figure 2 [FIGURE:2]). The external heating system must provide stable heat sources to the fluidized bed at constant power, maintaining furnace temperature between 550–600°C. The internal heating system requires frequent power adjustments to stabilize heating rod temperatures between 480–520°C. These complementary heating systems work in coordination to maintain the reaction zone temperature within the bed body at 300–340°C.

This research aims to develop a heating and control system suitable for denitrification fluidized beds to resolve current operational instability issues in both internal and external heating systems.

2 External Heating System Optimization

2.1 Failure Mechanism Analysis

The original heating furnace utilized a radiant heating design with aluminum silicate refractory fiber insulation (temperature resistance 1260–1450°C) that failed to provide adequate thermal insulation. This caused excessive temperatures in the furnace's metal exterior wall and junction box, leading to heating of copper terminal bolts, loosening of previously secure connections, and cable insulation damage through carbonization. These issues resulted in open circuits and short circuits in power lines, causing phase loss operation in the middle and lower furnace sections. Prolonged phase-loss operation under fault conditions caused heating wire burnout and external heating furnace damage. Additionally, during phase-loss operation, increased current in cables and terminals generated further temperature elevation.

Furthermore, the connection between nickel-chromium alloy wires and furnace wires at the fluidized bed external heating furnace terminals used stainless steel wire noses with cold riveting. When the furnace wire heated, this connection point experienced not only the wire's inherent temperature but also heat generated by current through contact resistance, causing the stainless steel wire nose to melt and the connection to burn out.

2.2 Conductive Furnace Design

Existing large-scale equipment heating furnaces primarily employ two types: radiant heating furnaces and conductive heating furnaces. Their performance comparison is presented below:

Table 1 [TABLE:1] Heating Furnace Comparison

Performance Metric Radiant Heating Furnace Conductive Heating Furnace Heating Principle Electric heating generates infrared radiation, primarily heat transfer by radiation Utilizes electromagnetic induction principle for rapid heating inside metal Energy Consumption High energy consumption, large heat losses High energy utilization, obvious energy-saving effect Temperature Control Good uniformity but slow response speed Precise control of heating zones, high temperature control accuracy Heating Rate Long heating cycle (dependent on radiation transfer rate) Rapid heating, improved production efficiency Material Suitability Suitable for high-absorption materials like carbon steel and alloy steel Obvious effect on conductive materials, not suitable for non-conductive materials

Based on the above analysis, conductive heating furnaces offer three key advantages: (1) Higher thermal efficiency and lower energy consumption. Conductive heating transfers heat directly through copper contact with the fluidized bed exterior, reducing losses from radiation through air or media. While radiant heating wastes energy through environmental heat dissipation, conductive heating significantly reduces losses through optimized insulation design, with thermal conductive cement further improving energy utilization. (2) More precise and uniform temperature control. Conductive heating surfaces can surround the fluidized bed for uniform heating, avoiding localized overheating or temperature non-uniformity caused by distance variations in radiant heating. By adjusting heat medium flow or temperature, conductive heating responds quickly to temperature changes, making it more suitable for field fluidized bed operating environments. (3) Enhanced safety and environmental adaptability. Conductive heating elements enclosed within the fluidized bed housing reduce burn risks. Based on this analysis, the fluidized bed external heating system was changed from radiant to conductive heating.

3 Internal Heating Rod Material Upgrade

3.1 Material Comparative Experiments

Abnormal operating conditions inside the fluidized bed can cause distributor plate clogging. When the distributor plate blocks, it affects the main control operator's judgment of the discharge endpoint. Under abnormal conditions, excessive discharge can cause bed wall caking to fall between heating rods, preventing heat dissipation. The temperature measurement points controlling the internal heating system are located at T3/T4 points in the fluidized bed, which cannot properly reflect abnormal temperature rises on heating rod surfaces. Simultaneously, caking prevents heat conduction from below the heating rods to the upper measurement points, causing slow bed temperature rise. The control program continues power supply, causing heating rod surface temperatures to increase continuously, resulting in metal plastic deformation. The metal surface forms oxide and decarburization layers through chemical reactions such as oxidation, decarburization, and hydrogen absorption. When heating rod heat dissipation is severely compromised, the internal heating wire fuses, causing heating rod open circuits.

3.2 Analysis of Heating Rod Materials

Based on fluidized bed operational experience, internal heating rods typically operate above 400°C. Due to uranyl nitrate solution and nitric acid production, heating rods easily experience creep, bending, or fracture in this environment, with severe surface oxidation and carbonization of stainless steel materials. Considering the special high-temperature acidic environment, internal heating rod materials must possess high-temperature resistance, corrosion resistance, high strength, and oxidation resistance. A comparison of various heating rod materials is presented in Table 2 [TABLE:2].

Table 2 [TABLE:2] Comparison of Stainless Steel Materials

Material Properties Characteristics Applications 316 Stainless Steel Alloy
Density: 7.98g/cm³
Melting point: 1370–1450°C
Yield strength ≥205N/mm²
Tensile strength ≥520N/mm²
Hardness (HB) ≤187 Good corrosion resistance, high-temperature strength, and good processability General chemical equipment 310S Stainless Steel Alloy
Density: 7.93g/cm³
Melting point: 1400–1530°C
Yield strength ≥205N/mm²
Tensile strength ≥520N/mm²
Hardness (HB) ≤187 Stable corrosion resistance and good oxidation resistance in high-temperature environments Automotive purification devices INCONEL 625 Alloy
Density: 8.4g/cm³
Melting point: 1290–1350°C
Yield strength ≥345N/mm²
Tensile strength ≥760N/mm²
Hardness (HB) ≤220 Excellent acid corrosion resistance (nitric, phosphoric, sulfuric, hydrochloric acids). Maximum service temperature 950°C. Hardens during long-term use at 550–700°C, reducing plasticity Engine manufacturing INCONEL 718/GH4169
Density: 8.24g/cm³
Melting point: 1260–1320°C
Yield strength ≥550N/mm²
Tensile strength ≥965N/mm²
Hardness (HB) ≤363 High tensile strength, fatigue strength, creep strength, and fracture strength at 700°C. High oxidation resistance at 1000°C Liquid fuel rocket engines

Based on the comparison above, Inconel718/GH4169 alloy was selected for internal heating rod surface material.

4 PID Control System Implementation

4.1 PID Algorithm Implementation

The current fluidized bed temperature control employs manual operation, where operators continuously adjust heating power based on internal bed temperature to achieve stable operation. However, human response speed is limited by operator judgment and execution time, making it difficult to address dynamic changes. Operator experience, fatigue, or distraction may lead to inconsistent regulation results, and emergency responses may be delayed or erroneous. Therefore, a PID program was considered to implement automatic fluidized bed regulation, offering the advantage of rapidly accepting input parameters, judging their values, and accurately outputting setpoints.

PID is a closed-loop feedback control algorithm that stabilizes fluidized bed system output quickly at setpoints by comprehensively regulating proportional (P), integral (I), and derivative (D) components. Its mathematical expression is:

$$u(t)= e(t)+ e(T)dT+$$

Where:
- $u(t)$: Control output (heating power)
- $e(t)$: Deviation between setpoint and actual value ($e = \text{setpoint} - \text{actual value}$)
- $K_p, K_i, K_d$: Proportional, integral, and derivative coefficients

In the fluidized bed temperature control system, the key measurement point temperature requirement (SV) in the bed body can be set and input into the controller. The controller calculates the error between the actual temperature value (PV) and SV, using this temperature error as the PID controller's input signal. After PID program calculation, the relevant signal for heater power adjustment is input to the power regulator, which outputs matching power to control heater temperature and thus the actual temperature inside the bed body. The error between the new PV value and SV is then input again into the PID controller for control. Through multiple PID control cycles, the error between PV and SV becomes progressively smaller, approaching SV until stability is achieved. The schematic diagram is shown in Figure 3 [FIGURE:3].

4.2 Distributed Power Compensation

The fluidized bed has experienced multiple temperature anomaly events caused by heating rod caking during operation. Continuous operation under abnormal conditions can easily lead to heating rod burnout. To reduce heating rod failure rates, over-temperature protection is required—when temperatures rise abnormally, the heating rod's power must be reduced to lower its temperature. However, reducing heating rod power causes internal bed temperature to drop, failing to meet operational requirements.

Therefore, a power compensation program was developed for the heating rod control system with the following implementation steps:

4.2.1 Heating Rod Material Upgrade

The heating rod surface material was upgraded to Inconel718/GH4169 alloy, with thermocouples added inside each rod. These thermocouples connect to the control system, enabling real-time monitoring and temperature protection control for each individual heating rod.

4.2.2 Additional Internal Heating System Control Cabinet

The internal heating rod control system is installed in a local control cabinet that regulates bed temperature through power adjustment of fluidized bed internal heating rods. Temperature measurement points on the heating rods serve as controlled signals input into the control system, with individual over-temperature alarm protection functions. The local control cabinet features a reliable, intuitive human-machine interface enabling convenient local start/stop of heating rods, manual or automatic temperature adjustment, switching between manual and automatic power regulation, local/remote control switching, and current and temperature monitoring functions.

The 28 heating rods are divided into three rings from inside to outside (see Figure 4 [FIGURE:4]), controlled by five power regulators. Each thyristor controls 4–6 heating rods, with independent circuit breakers for each rod and current transformers in each circuit. Through logical judgment and over-temperature alarm functions in the local control system, fault alarm signals can be issued for each individual heating rod. Upon detecting a fault signal, the heating rod triggers an over-temperature alarm, indicating possible abnormal conditions in that zone while simultaneously notifying the main control to reduce that heating rod group's power by 10% and increase adjacent groups' power, thereby protecting heating rods while ensuring no heat loss in the fluidized bed.

5 Experimental Tests

5.1 External Heating Furnace Individual Test

After installing the new heating furnace, a heating test was conducted on the fluidized bed external heating furnace, with comparative data collected under identical conditions with the old furnace:

Table 3 [TABLE:3] Comparative Tests of External Heating Furnace for Fluidized Bed

Parameter Old Furnace New Furnace External Heating Upper Section Heating power percentage 42L/h 44L/h Furnace temperature (°C) Current (A) External Heating Middle Section Heating power percentage Furnace temperature (°C) Current (A) External Heating Lower Section Heating power percentage Furnace temperature (°C) Current (A) Key Bed Temperature Points Heating power percentage Temperature (°C) Current (A) T3 point temperature (°C) T4 point temperature (°C)

Test data demonstrate that the new heating furnace meets operational temperature requirements with lower heating power and higher thermal efficiency compared to the old furnace.

5.2 Heating Element Unit Test

Two new heating rods were randomly selected for resistance measurement and comparative testing against original fluidized bed heating rods, connected to power regulators for heating tests at different power levels. Test data are presented below:

Table 4 [TABLE:4] Comparative Test Data on Temperature Rise of Heating Rods

Parameter Comparison Rod 1 (1.5kW) Test Rod 2 (2kW) Test Rod 3 (2kW) Current (A) Voltage (V)

Figure 6 [FIGURE:6] Temperature Curves from Comparative Tests of Electric Heating Rods

After starting the fluidized bed external heating and reaching 310°C in the reaction zone, internal heating rod power tests were conducted. The set temperature (>320°C) was configured on the control cabinet touchscreen. During heating, internal heating power was set to 25%, 50%, 75%, and 100% sequentially, with current measured and recorded for each internal heating rod.

Table 5 [TABLE:5] Record of Temperature Variation with Power for Internal Heating Rods

Power Level 25% Power 50% Power 75% Power 100% Power Heating rod current (A)

Figure 7 [FIGURE:7] Temperature vs. Power Curves for Internal Heating Rods in Fluidized Bed

Test data indicate that the new heating rods demonstrate improved heating performance without abnormal conditions under various power levels, meeting field usage requirements.

6 Analytical Framework for Stability Enhancement in Fluidized-Bed Heating Systems

After establishing test conditions, the frequency of five heating rod groups was manually adjusted through the field control cabinet to observe temperature variation ranges at key fluidized bed points, with results compared against pre-adjustment temperatures:

Table 6 [TABLE:6] Record of Temperature Variation with Power for Internal Heating Rods

Test Condition Parameters Group 1 Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C) Group 2 Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C) Group 3 Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C) Group 4 Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C) Group 5 Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C)

After commissioning, a heating rod exhibited temperature elevation during operation. Key fluidized bed temperature variation ranges were observed and compared against pre-change temperatures, with data recorded as follows:

Table 7 [TABLE:7] Adjustment Data Under Actual Operating Conditions

Test Condition Parameters Automatic Temperature Compensation Test
Compensation value: 10%
Voltage: 52.5% Pre-compensation temperature (°C) Post-compensation temperature (°C)

Comparative analysis of manually adjusted and actual operating condition data demonstrates that the temperature regulation system operates accurately, stably, and reliably, meeting normal process operation requirements.

7 Conclusions

Through this fluidized bed stability research study, three key improvements were implemented: optimization of the fluidized bed heating furnace heating method, upgrade of heating rod materials, and enhancement of the fluidized bed control system. Experimental verification confirmed:

1. The new heating furnace meets functional requirements with slightly better heating performance than the old furnace.
2. The optimized internal heating rods demonstrate superior heating performance compared to original rods. Manufacturing process upgrades reduced heating rod failure rates, and real-time temperature monitoring of each individual rod prevents abnormal operating conditions.
3. When the temperature compensation program in the control system activates, it effectively reduces power to abnormally heated rods while maintaining reaction temperature requirements inside the bed body, thereby improving overall fluidized bed system stability.

This research, targeting "precision, efficiency, and reliability," improved the existing fluidized bed heating system through technological innovation and engineering practice. Validation confirmed significant enhancement of fluidized bed heating system stability, providing valuable experience for stable operation of denitrification fluidized beds.

References

[1] Chao Wen, et al., "A Control Method and System for Post-Treatment Fluidized Bed," Application Number: 202311269818
[2] Qiang Li, et al., Principles and Applications of Fluidization [M]. Xuzhou: China University of Mining and Technology Press, 1994
[3] Libing Yao, "Development Trends of Fluidized Beds" [J]. Pharmaceutical Engineering Design, 1989(6), 16-19
[4] Aiguang Lin, Fundamentals of Chemical Engineering [M]. Beijing: Tsinghua University Press, 1999
[5] Guoqiang Jiang, "Application of Temperature Control Systems in Fluidized Beds" [J]. National Defense Industry Press, 2003
[6] Jiaqian Le, Instrument Technician Handbook [M]. Beijing: Chemical Industry Press, 2003: 232
[7] Baoqi Yuan, Principles and Design of Heating Furnaces [M]. Aviation Industry University Press, 1989-06

Note: Funding for the demonstration plant denitrification fluidized bed heating system stability research project was provided by the Gansu Provincial Department of Science and Technology High-Skilled Talent Funding Program.