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Article

Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge

by
Jianqiao Yang
1,
Shijing Xie
2,* and
Shuzhong Wang
1
1
Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an 710049, China
2
School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(12), 5101; https://doi.org/10.3390/app14125101
Submission received: 16 May 2024 / Revised: 4 June 2024 / Accepted: 4 June 2024 / Published: 12 June 2024
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Abstract

:
Supercritical water oxidation is a promising technology for decomposition of industrial wastewater and sludge. However, the system is operated under high temperature and pressure (usually higher than 500 °C and 25 MPa). Corrosion of component materials and salt deposition may lead to leakage or even the burst of the pressure vessel in the SCWO system, resulting in a high level of worry about the safe operation of the system. In this paper, the safety management and accident- control strategies are introduced according to a commercial-scale SCWO plant in China. The safety management strategy refers to the special design and operation strategies of some facilities. Different types of potential accidents are analyzed and the coping strategies for accidents of different levels of severity are described in detail. The strategies are valuable for the safe operation of commercial SCWO plants and other plants operated in high temperature and high pressure.

1. Introduction

Sludge is a by-product of wastewater treatment. The amount of sludge has been boosted with the expansion of the discharge amount of wastewater, including both municipal sewage sludge and industrial sludge [1]. Traditional sludge treatment methods are landfilling and incineration [2,3]. Landfilling is becoming unacceptable with stricter environmental policies. Incineration requires a pre-drying procedure and combustion aids to guarantee stable combustion, which increases the operating costs. Moreover, the gaseous products of incineration, including NOx and dioxin, may cause secondary pollution [4].
Supercritical water oxidation is a powerful organic-waste treatment method that can completely decompose the organic materials without a pre-drying procedure and secondary pollution [5,6,7]. Compared with other advanced oxidation technologies, there are several advantages to SCWO. Firstly, supercritical water (SCW) acts as a nonpolar solvent, which is able to be completely miscible with oxygen and organic materials. Therefore, intense oxidation reactions occurred in SCW, transforming the organic matters into N2, CO2, H2O, and inorganic salts in a short time [8]. Secondly, the strong heat release owing to the intense oxidation reaction is able to preheat the cold feedstock to a supercritical state again. That is to say, there are no extra heating resources needed during the normal operation of an SCWO system, which effectively saves the running cost [9]. Thirdly, the COD removal coefficient by the SCWO process is extremely high. For instance, 99.9% of TNT can be decomposed in SCW at 500 °C for 120 s [10]. The removal efficiency of polyfluoroalkyl substances can reach 99.999% after only ~30 s reactions in SCW at a temperature higher than 610 °C [11].
Many commercial companies constructed industrial SCWO plants for organic waste treatment [12,13,14], such as General Atomics [15], SRI International [16], Innoveox [17] and Hanwha [18]. However, some of the plants were forced to shut down due to the severe corrosion of component materials or blockage of pipelines caused by salt precipitation, which are the two obstacles impeding the commercialization of the SCWO technology [19]. Meanwhile, there are some new industrial SCWO plants under construction, such as the 50 t/d SCWO plant for sludge in China, which is designed by our research group [20,21].
Since the SCWO concept was proposed by Modell in the 1980s, many aspects related to this technology have been studied, including reaction kinetics [22,23,24], corrosion behavior of construction materials [25,26], salt precipitation problem [27], reactor design concepts [28], system design [29] and low-cost start-up strategies [30]. These studies mainly focus on reaction mechanisms of different kinds of organic matters, dissolution and precipitation behaviors of inorganic salts, and corrosion mechanisms of alloys in SCW. However, few articles are focusing on the safety concept of the SCWO system.
The SCWO process can be considered as the first system that is operated in a high-temperature and high-pressure environment with highly corrosiveness, high abrasiveness, and a high flow-rate feedstock. There are many potential safety risks during the operation of the SCWO plants; for example, unpredictable bursts of pipelines caused by corrosion, pipeline blockage caused by salt precipitation, and flammable- and combustible-gases leakage caused by valve failures. Therefore, safety management and accident control are vital for an SCWO plant, and can effectively avoid damage to equipment and loss of property.
Currently, a commercial-scale SCWO plant for mixed sludge is under construction in China, designed by our research group. The novel design concept applied in this plant has been introduced in our previous articles [20]. In this paper, the safety management and accident control strategy for this commercial SCWO plant is introduced. The unique features of the SCWO system that related to safety scope are analyzed. the special design of facilities and operating methods to avoid accidents is introduced. Different types of potential accidents are analyzed, and coping strategies for accidents of different orders of severity are described in detail. This information is valuable for the safe design of commercial SCWO plants and other plants operated in high temperature and high pressure.

2. Unique Characteristics of the Feedstock for SCWO Plants

Usually, an SCWO process is operated under high-temperature and high-pressure conditions of up to 600 °C and 30 MPa [31]. The industrial process operating in similar conditions contains supercritical water reactors (SCWR, one of the potential next-generation nuclear plant concepts) [32,33] and supercritical water gasification (SCWG, a hydrogen production technology). Table 1 shows a typical concentration of corrosive ions and oxygen in the feedstock of each process. It can be seen that the concentration of oxygen and chloride ion in a typical SCWO system is about 106 times and 109 times higher than that in a SCWR system, forming a much more severely corrosive environment. In this section, the unique characteristics of the feedstock of SCWO systems are introduced, and the potential risks caused by the feedstock are analyzed.
The potential risks caused by organic wastewater and sludge are different; therefore, they will be discussed separately.

2.1. Wastewater

The SCWO reaction of many types of organic wastewater has been investigated, such as pharmaceutical wastewater [34], coal chemical wastewater [35] and dyeing wastewater [36]. No matter what kind of wastewater, the chemical oxygen demand (COD), chemical composition and pH value of the wastewater should be detected and measured at first, which can significantly affect the operation of an SCWO system.
COD refers to the concentration of organic matter. Organic matter in any concentration can be dissolved in SCW, but usually the COD of the feedstock for the industrial SCWO plant should not be too high (e.g., less than 120,000 mg/L). In order to improve the oxidation efficiency, excess oxygen was injected into the system to react with the organic matters. An extremely high COD of feedstock means a terribly high concentration of oxygen would be injected into the reactor. As a result, once the feedstock and the oxygen mixes together, a large amount of heat will be released due to the drastic oxidation reaction, leading to the over-heating of the reactor. Moreover, a high value of the local oxygen concentration may lead to severe corrosion of reactor materials.
The chemical composition of feedstock is another parameter that needs to be paid attention to. Although the main target of the SCWO plant is to decompose the organic matter in the wastewater, some wastewater also contains considerable amounts of inorganic salts, such as K+, Na+, Cl− and SO42−. The solubility of these inorganic salts decreased to thousands of ppm in SCW, separating out and depositing on the inner wall of equipment or pipelines. With the accumulation of the salt deposits, the pipelines could be totally blocked and the SCWO system would be forced to shut down. The most important equipment in an SCWO system for salt precipitation is the pre-heater or the heat exchanger, because the wall temperature of these vessels is the highest. Salt deposits crystallize once the saturation point is reached, and the crystalline salt will stick onto the inner wall of the vessels and result in the plugging of the equipment in several minutes. Moreover, the polymerization of the organic materials in an oxygen-free environment (heat exchanger) may happen, forming insoluble char or tar and resulting in scaling of heat-transfer surfaces or plugging [37].
An acidic feedstock would improve the risk of severe corrosion of heat exchangers. Many research studies show that the corrosion of constructional materials in a subcritical environment is higher than that in supercritical water [38,39,40]. This is due to the solubility of the corrosive ions, such as H+ and Cl−, reaching the highest value in a near-critical temperature environment. For example, Asselin et al. [41] found that the corrosion rate of stainless steel 316 can reach 340 mm/year in subcritical water containing oxygen, sulfuric acid and ammonium ions. An acidic feedstock is definitely a disaster for an SCWO plant and it should be strictly avoided. Considering the fact that some oxidation reactions of organic matters also produce acids, the pH value of the effluents should also be taken into account because the effluents also go through a subcritical state.
There are many nitrogen-containing organic materials in wastewater to be treated by the SCWO process. The nitrogen element in organic matters can be directly transferred into N2 by an SCWO reaction at a temperature higher than 700 °C or with the aid of a catalyst [42]. However, most of the commercial SCWO plants were operated with the highest temperature of less than 650 °C, to decrease the equipment investigation [43]. As a result, the nitrogen element in organic matter is converted to ammonia gas and is dissolved in effluent. The ammonia gas is toxic and has a pungent smell, which is a risk for the health of the operating personnel.

2.2. Sludge

Sludge is produced from the organic-wastewater treatment system by biological processes or physical–chemical processes. The potential risks caused by wastewater also exist in sludge. Additionally, there are more potential risks caused by the unique features of sludge.
The most significant difference between the sludge and the wastewater is that some solid-phase materials are contained in sludge, including some organic materials such as carbohydrates, proteins, lipids and lignin, and some inorganic materials such as sand and ash. The existence of insoluble solid particles is harmful to the system, causing wear of the seal structures of valves and pumps. Some insoluble solid particles with a relatively high diameter may precipitate in the system, blocking the pipelines. Moreover, the viscosity of sludge is obviously larger than that of wastewater, which increases the difficulty for transport.

3. Safety Management Strategy

In this section, the safety management strategies for our commercial-scale SCWO plant are introduced, including some special design concepts for equipment and system flows. As introduced before, the biggest challenge for an SCWO system is corrosion and salt deposition. Therefore, the safety management of the system is established around how to avoid severe corrosion and salt deposition, and how to minimize the damage caused by the irregular operation of the system when severe corrosion or salt deposition occurs.

3.1. Temperature Management of the Reactor

There is no doubt that the facility with the highest temperature in an SCWO system is the reactor, which is the place that oxidizing agents and organic materials mix and oxidation reaction occurs. The over-heating of a reactor is a severe accident that may cause a fast-corrosion failure of a reactor. Moreover, the allowable stress of the alloy used as component materials for a reactor decreases with the increasing of temperature. The most widely used material for the reactor is Ni-based alloy 625. The allowable stress of Alloy 625 at 650 °C is about 140 MPa, but it rapidly decreases to 80 MPa at 700 °C. That is to say, over-heating of a reactor is very dangerous because the component materials cannot withstand the high pressure (up to 30 MPa) at the temperature beyond the design value, resulting in swellings of tubes or even bursting.
In our commercial SCWO plant, two methods are used to prevent the over-heating of a reactor. The first one is oxygen multipoint injection. Figure 1 is a simplified SCWO system diagram, which includes only the core facilities such as the pre-heater, the heat exchanger and the reactor. For a conventional SCWO plant, oxygen is injected only into a reactor. In our new plant, there are many oxygen injection points located at the heater, the heat exchanger and the reactor, as shown in Figure 1. A small amount of oxygen is injected into the heat exchanger to perform the oxidation reaction with a portion of organic material. The injecting point on the heat exchanger is located to the place where the temperature of the feedstock is about 374 °C, which is considered as the “large specific-heat region” [44]. By injecting a portion of oxygen into the heat exchanger and the heater, both the COD of feedstock and the amount of oxygen to be injected into the reactor can be diminished, which effectively reduces the intensity of the oxidation reaction in the reactor. Moreover, as shown in Figure 1, there are many oxygen-injection points that are uniformly distributed from the entrance to the end of the reactor. Different amounts of oxygen are injected from these entrances, which can also decrease the intensity of the oxidation reaction at the beginning of the reactor and reduce the risk of over-heating. Some studies show that the multi-injection of oxygen has better results than the single injection [45,46]. A novel design mixer was used for the mixing of the oxygen and the feedstock to avoid the formation of hot spots. The structure and the function of the mixer has been introduced in our previous article [20].
Oxygen multipoint injection is a method for reducing the potential risk of over-heating of the reactor, which cannot be completely avoided. Therefore, a desuperheater is employed for over-heating accidents. As shown in Figure 1, the desuperheater is located on a bypass of the main feedstock pipeline from the heater to the reactor. The valves V1 and V2 are kept closed under the normal operating condition. If the temperature of the reactor increases sharply in a short time, valves V3 and V4 are closed, valves V1, V2 and V5 are opened, and cooling water is injected into the system via the desuperheater. The flux of cooling water is calculated according to the real-time highest temperature of the reactor, aiming to reduce the temperature to a normal value. The bypass is used to avoid the insoluble solid particles from feedstock blocking off the tiny holes on the desuperheater (namely the outlet of cool water) during normal operation.

3.2. Salt Deposition Management of the Heater

The start-up process of an industrial SCWO plant is a very challenging step, since the inorganic salts in feedstock are likely to precipitate on the inner walls of the heating modules [28]. During a start-up process, the electric heater is the only heating source of the system, which contains several tubes surrounded by heating modules. In a heater, the feedstock is heated from room temperature to supercritical temperature. The pipe walls of the electric heater are directly heated. Inorganic salts in feedstock are easier to crystalize near the pipe walls, because the temperature of the inner wall is the highest. As a result, the heater is likely to be blocked by salt precipitating during a start-up process. Some efforts should be made to minimize the risk of salt deposition.
A novel heater made especially for an SCWO system was developed by our research group. The structure of the heater is shown in Figure 2. The core components of the new heater are similar to those of the traditional one, namely, some tubes and surrounding heating modules, but there are several special designs to solve the salt deposition breakdown. Firstly, the new heater is divided into many parallel units, and each unit contains only one tube. The temperature and pressure of the inlet and the outlet of every unit is measured, and the pressure difference between the inlet and the outlet of every unit is automatically calculated and used for evaluation of the degree of blockage. By this method, the heating process of the feedstock from room temperature to supercritical temperature can be divided into separated parts. If the heater is blocked due to salt deposition, the operating personnel can easily find out where the salt precipitation happens by the difference of pressure between the inlet and the outlet of each pipe.
Secondly, the length of the heating modules that are fixed on the tubes is only a half of the whole length, and the remaining surface of the tube is covered by thermal insulation materials. The heating module surrounding a half part of the surface is placed on a sliding rail, and it can be moved along the lengthwise direction of the tube easily. The thermal insulation material on the tube is dismountable. When the salt deposition occurs in one tube the thermal insulation material is dismounted, and the heating module is moved to the other side of the tube. Therefore, the salt deposition area in the tube can be quickly cooled in air. The precipitated salt dissolves again when the temperature of feedstock in the block area decreases below the saturation point of the salts. It is notable that the movable heating modules and the dismountable thermal insulation material are only installed on the tubes where salt deposition is likely to occur.
Apart from the two methods introduced above, a new ultrasonic descaling technique is under development by our research group. Ultrasonic cleaning is an efficient method for removing the fouling layer by destroying the surface morphology of inorganic salts via the acoustic cavitation effect [12,47], which is widely used for the descaling of heat transfer equipment [48]. Moreover, ultrasonic treatment is an eco-friendly method for the disposal of sludge [49,50]. As shown in Figure 2, some ultrasonic working heads are welded onto the tube. During a start-up process, the ultrasonic generators are turned on to decrease the risk of salt precipitation. However, the obstacle for using the ultrasonic facilities is that the function distance of a working head is limited. Only when a large number of ultrasonic working heads are installed on one tube can a satisfactory descaling effect be achieved. The descaling technique is still being investigated by our research group, and some results on the effect of ultrasound on delaying the salt deposition and improving the removal efficiency of the sludge will be published in the future.

3.3. Water Tank for Pressure-Relief-Valve Discharging

An SCWO system is operated under high pressure. In our commercial plant, pressure relief valves are installed on the heat exchanger and the reactor to protect the vessels by venting fluids when serious overpressure occurs. In general, the discharged fluids from a pressure relief valve are recommended to blow directly into the atmosphere. However, the discharging fluid from the heat exchanger mainly contains unreacted organic matter because the feedstock has not been mixed with oxygen. The discharging fluid from the reactor is more complicated, containing reaction products such as CO2 and NH3, and partially oxidized products such as H2S, olefin, phenols, pyrrole, and so on. Moreover, a considerable amount of unreacted oxygen is also discharged with the effluent. The toxic, flammable and strong-smelling effluent should not be directly discharged into the atmosphere; therefore, a tank is used to collect the discharging fluids.
Figure 3 shows the schematic of the tank for pressure-relief-valve discharging. High-temperature fluid is ejected from the effluent ejector via the multi-nozzles; meanwhile, cooling water is ejected from the spray. There is a gas outlet located at the top of the tank, and a fluid outlet at the bottom. Due to the special characteristics of the discharging fluids, some special designs are used to guarantee safety during the relief-valve discharging process.
The effluent ejector contains three pipes, and there are many nozzles on each pipe, as shown in Figure 3b. The multi-nozzle structure is used to divide the effluent into many parts and obtain a better heat exchanging effect between high-temperature effluents and cooling water. It is notable that there would be a considerable amount of oxygen contained in the discharging fluid if the pressure relief valve on the reactor opened. There is an in-tank fire risk when the discharging fluid is injected into the tank via the nozzles because the high-velocity oxygen rubs dangerously with the surface of the nozzles. To avoid the ignition of fire and the ensuing explosion in the water tank, Monel 400 (Ni-Cu alloy) is used as the component material of the effluent ejector.
The temperature and pressure of the discharging fluids that are ejected from a pressure relief valve can be up to 600 °C and 30 MPa. Once a pressure relief valve opens, the pressure of the discharging fluid drastically decreases, resulting in the formation of steam. At the bottom of the tank, some water at room temperature is stored. The nozzles from the effluent ejector are not directly in contact with the stored water, to avoid water hammer when the steam dissolves in water. By this means, the solid and liquid materials in the high-temperature discharging fluid that is ejected from the nozzles are able to mix with the stored water. The gases from the discharging fluid, including soluble gases and insoluble gases and steam, are cooled down by the cooling water from the cooling water spray at the middle of the tank. Moreover, there is a coil heat exchanger located above the tank. The high-temperature gases can be totally cooled down by the heat exchanger, and all the steam can be transferred into liquid phase in the coil heat exchanger and flow back to the tank. Insoluble gases are introduced to a biofilter system, which is a highly efficient separation process for removing organic matter in the gas phase.

4. Accident Control Strategy

Figure 4 shows the schematic diagram of the commercial SCWO plant for sludge. The accident control strategy is introduced according to this “new” schematic diagram. It should be noted that this schematic diagram and the older one from our previous publication [20] describe the same system, but the emphasis of the two article is different. The older article [20] places emphasis on the novel design concept of the commercial SCWO plant, including the process flow and how the specific function of the system can be achieved. This paper focuses more on the accident control strategy for the commercial system. As a result, the pipelines and valves specific to the accident management concept are exhibited in the new schematic diagram; on the other hand, some less important components are not shown.

4.1. Classification of the Accident and the Potential Damage Analysis

For an SCWO system, the concept of “accident” is defined as every factor that affects the regular operation of the system. Among these factors, some low-risk accidents such as the failure of some mechanical components can be solved by switching the backup equipment, but some accidents such as the blockage of the heat exchanger may lead to shut down. In this section, the possible accidents that may happen in the commercial SCWO plant for sludge are analyzed, and the potential damage of different accidents is evaluated and grouped into three grades. All the possible accidents, with different risk grades, are summarized in Table 2.

4.1.1. Level 1 Accident

A Level 1 accident can be considered as an “incident” that is not very serious. It refers to the incidents that are caused by the failure of the replaceable equipment and do not affect normal operation. For example, all the pumps in the system have a parallel connection backup which can be switched in a short time if the working one is broken. Some facilities, such as the three-phase separator, have a motor, which is also likely to break down. For these facilities, although there is no parallel facility in the system, the breakdown can be solved by replacing the broken component without the shutdown of the whole system. However, some equipment cannot be restored during the operation of the system, such as the failure of the electric heater. Therefore, it cannot be considered as a Level 1 accident.

4.1.2. Level 2 Accident

An SCWO system is operated in a high-temperature and high-pressure environment. The maintenance of high pressure in the system is of great importance because the loss of pressure of the high-temperature fluids means they can drastically vaporize. The SCWO system is series-connected; therefore, the blockage or leakage at any place can affect the whole system. A Level 2 accident refers to an accident with limited impact on the normal operation of the system. If a Level 2 accident happens, the normal operation of the system can be interrupted, and it cannot be resumed by simply replacing some components. For instance, the opening of a relief valve from a high-temperature and high-pressure facility is a Level 2 accident, because the relief valve re-seats and the pressure of the whole system can be established again after discharging some of the feedstock. Some action should be taken to minimize secondary damage to the facilities that is caused by a Level 2 accident, and the whole system should be shut down immediately.

4.1.3. Level 3 Accident

The main difference between a Level-3 and Level-2 accident is that the high pressure environment of a system cannot be recovered anymore in a Level 3 accident. For instance, the burst of pipelines or damage to facilities that operate in a high-temperature and high-pressure environment is a Level 3 accident, because the leakage of the feedstock cannot be controlled, and the series connection of the SCWO system is cut off. If a Level 2 accident happens, the most important strategy is to prevent fires, bursts and casualties.

4.2. Core Principles for Accident Control

The response methods for different kinds of accident are different. However, there are two core principles for accident control, and they are applicable to the response methods for all kinds of accidents.
(1) The oxygen system should be cut off immediately once an accident happens. Oxygen is a combustion-supporting gas. Considering the fact that there are many kinds of flammable organic matter in feedstock, the quantity of oxygen in the system should be strictly limited. Once an accident happens, the regular oxidation reaction cannot be maintained. Therefore, the oxygen that is injected into the reactor cannot be thoroughly consumed, resulting in an increasing of the concentration of oxygen in the effluent. Moreover, the effluent may flow backward to oxygen pipelines if an overpressure accident happens, resulting in the pollution of the oxygen pipeline. It is notable that the oxygen pipeline should be kept scrupulously clean during the transportation of oxygen to prevent ignition of fire.
(2) The flow of feedstock in the system should not be stopped in any circumstance. The feedstock of our commercial-scale SCWO system is sludge. As introduced in Section 2.2 and the previous article [20], there are considerable amounts of insoluble solid particles in sludge. The solid particles are surrounded by some organic matter at first. After SCWO reactions, they are separated from the organic matter, and flow with the effluent. The flow rate of feedstock should be kept at a relatively high value to guarantee that the solid particles can be carried with the bulk fluids. Otherwise, the solid particles may accumulate in pipelines or facilities, leading to blockage of the system. If the flow of feedstock is interrupted and the system cannot be resumed because of accidents, the high-temperature and high-pressure sludge feedstock should be discharged from the system. The methods to discharge the feedstock from the system corresponding to a Level 2 accident and a Level 3 accident are different, and will be introduced in the next section.
The two rules mentioned above should be strictly obeyed; otherwise, irreversible damage to the facilities, or even casualties, could be caused by some accidents.

4.3. The Movement toward Accident Countermeasures

In this section, the movement toward accident countermeasures is introduced, based on Figure 4.

4.3.1. Level 1 Accident

A Level 1 accident refers to the failure of some mechanical equipment. The strategy for dealing with a Level 1 accident is as follows:
(1)
There are several pumps running in the system, and all the pumps have a backup. If an ongoing pump is broken, the movement is to switch to the backup one. Then, the system enters the normal shutdown procedure, because every pump in the system is indispensable. For the sake of guaranteeing safety and preventing potential damage, the system should not run without a backup pump.
(2)
If the motor of the three-phase separator (9) is broken, the solid phase in the effluent cannot be separated with the liquid phase anymore. To handle this accident, the effluent from the liquid-phase outlet of the three-phase separator (9) is transported to the underground tank (16) for storing, and the system is enters the normal shutdown procedure.

4.3.2. Level 2 Accident

The strategy for dealing with a Level 2 accident is to make the system shut down immediately, by a so-called “fast shutdown procedure”. There are some differences between the normal shutdown procedure and the fast shutdown procedure. For the normal shutdown process, the feedstock of the system is changed from sludge to water gradually, by adding water into the sludge tank (1). The temperature of the reactor (7) is gradually decreased with a decreasing rate of 100 °C per hour, which is achieved by interlocking the temperature signal from the reactor (7) and the power of the electrical heater (5). According to our estimation, it takes about 6 to 8 h for the system to shut down by the normal procedure. Obviously, there is not so much time during the occurrence of a Level 2 accident. The “fast shutdown procedure” is aimed at stopping the system and replacing the sludge in the system in one hour. The sludge tank (1) is directly cut off from the system, and the water from the water tank (2) is directly transported into the system. The heater (5) is turned off, and the decreasing rate of the reactor temperature is not controlled anymore. The effluent is discharged into the underground tank (16) for storing.
Although the “fast shutdown procedure” can be applied to handle every Level 2 accident, there are some special operations for different accidents.
(1)
Overpressure accident: if overpressure occurs in the electrical heater (5), the first step is to change the location of the heating module where salt precipitation forms, as introduced in Section 3.2. If the overpressure occurs in the heat exchanger (4) or the reactor (7), the fast shutdown procedure should be carried out directly.
(2)
Over-heating accident: the first step in handling an over-heating accident is to cool down the place where the temperature is exceeding the normal. If it occurs in the heater (5), the heating module of the heater (5) should be turned off immediately, and all the dismountable thermal-insulation material should be removed to help cooling. If over-heating occurs in the heat exchanger (4) or the reactor (7), the desuperheater (6) is used for fast cooling, and then the fast-shutdown procedure will be carried out.
(3)
Opening of pressure relief valves: the opened relief valve will re-seat when the pressure decreases, back to the set-point. During the period of the relief valve opening, the feedstock pump (3) should be stopped. The insoluble solid particles from the feedstock may stick to the sealing face of the relief valve when it closes, leading to the unstoppable discharging of the feedstock. Therefore, the valves from the inlet and the outlet of the overpressure equipment should be closed once the relief valve opens. For example, if the relief valve S3 opens, the valves V6 and V9 should be closed immediately to isolate the reactor (7) from the system. When the relief valve closes, the fast-shutdown procedure will be carried out.
(4)
Breaking of the heater (5): there is no backup for the heater (5) in the system, and the heater (5) functions by adjusting the reaction temperature during normal operation and the normal shutdown procedure. If the heater (5) cannot work anymore, the system should enter into the fast-shutdown procedure.

4.3.3. Level 3 Accident

The core principle for dealing with a Level 3 accident is to eject the sludge feedstock, and then use large amounts of water to clean the pipelines and facilities. If there is no electricity supply, the ejecting and cleaning process is hard to achieve.
(1)
Pipeline-Leak Accident
The rupture of pipes is owing to the corrosion of component materials. Differently from the accident caused by the opening of the relief valve, the rupture of a pipe leads to the irreversible pressure loss of the system. As a result, large amounts of feedstock are ejected from the cracking place. To handle this issue, the valves at the inlet and the outlet of the pipe should be closed immediately. Meanwhile, the valves V3, V4, V6, V9 and V10 are closed, to divide the system into three high-pressure parts, as shown in Figure 4. The next step is to discharge the high-temperature and high-pressure feedstock and effluent to the underground tank (16), slowly opening the valves V11, V12 and V13, one by one, releasing the feedstock from every part. The opening degree of valves V11, V12 and V13 can be controlled, and the decreasing rate in the pressure of each part is controlled to about 5 MPa/min. When the discharging process is finished, the valves V11, V12 and V13 are closed and the valves V3, V4, V6, V9 and V10 are opened; then the pump (3) is started, to transport cool water into the system to clean pipelines and facilities.
(2)
Power Outage Accident
Generally, a power outage accident could not happen in the commercial SCWO plant because it is equipped with a dual power-supply system. However, some extreme situation such as an earthquake, typhoon or flood may lead to the failure of both ways of power supply. Therefore, a special process is designed to handle the power outage accident.
The first step is to close the valves V3, V4, V6, V9 and V10 by hand, to divide the system into three high-pressure parts. Then, slowly open the valves V11, V12 and V13 to discharge the high-temperature and high-pressure feedstock and effluence into the underground tank (16). This procedure should be carried out by hand because of the outage of electric power. Finally, open the valves V15, V16 and V17 to let the cooling water from the overhead water tank (15) clean the pipelines and facilities. The cleaning water is injected into the system via the entrance after the pump (3). Finally, the cleaning water flows in o the underground tank (16) for storing.

5. Conclusions

In this paper, the potential risks, safety management and accident-control strategy are introduced according to a commercial-scale SCWO plant in China.
The main risk of the SCWO system is overpressure in heaters and over-heating in reactors. Some special equipment and system flows are used in our commercial SCWO plant for safety management. To prevent the over-heating of the reactor, oxygen injection points are set at many places, including the heater and the heat exchanger and in many parts of the reactor. To settle the overpressure problem of the heater caused by salt precipitation, a heater with slidable heating modules and dismountable thermal-insulation materials is used. A special design tank is used to collect the toxic, flammable and strong-smelling discharging fluids when a pressure relief valve opens.
The accidents that may happens in an SCWO system are divided into three levels, based on potential damage. The Level 1 accidents refer to the incidents that can be solved by switching to backups. The strategy to deal with a Level 2 accident is to make the system stop running immediately, by the fast-shutdown procedure. If a Level 3 accident happens, the core principle is to eject the sludge feedstock from the system and then use large amounts of water to clean pipelines and facilities.
The SCWO system is a very special industrial process that involves high temperatures, high pressure, and corrosive feedstock. The potential risks, safety management and accident-control strategy introduced in this study is valuable for the safety design of commercial SCWO plants and other plants operating under high temperatures and high pressure.

Author Contributions

Formal analysis, S.X.; investigation, J.Y.; writing—original draft preparation, J.Y.; writing—review and editing, S.X.; supervision, S.W.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Projects from National Natural Science Foundation of China [52176162].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A simplified SCWO system diagram for showing especially the principle of temperature management of the reactor.
Figure 1. A simplified SCWO system diagram for showing especially the principle of temperature management of the reactor.
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Figure 2. Schematic diagram of the heater in the SCWO system.
Figure 2. Schematic diagram of the heater in the SCWO system.
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Figure 3. Schematic diagram of the tank for pressure-relief-valve discharging: (a) is the front view, and (b) is the top view of the ejecting pipes of the discharging fluids.
Figure 3. Schematic diagram of the tank for pressure-relief-valve discharging: (a) is the front view, and (b) is the top view of the ejecting pipes of the discharging fluids.
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Figure 4. Schematic diagram of the commercial SCWO plant for sludge. This diagram focuses more on the accident control strategy for the system. Some less important pipelines and facilities are not shown.
Figure 4. Schematic diagram of the commercial SCWO plant for sludge. This diagram focuses more on the accident control strategy for the system. Some less important pipelines and facilities are not shown.
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Table 1. Typical concentration of corrosive ions and oxygen in the feedstock of an SCWO system, a USC boiler and an SCWR system.
Table 1. Typical concentration of corrosive ions and oxygen in the feedstock of an SCWO system, a USC boiler and an SCWR system.
ItemsUSC BoilerSCWRSCWO
pH value>9.3>9adjustable
Dissolved oxygen12–18 ppb<50 ppb1000–100,000 ppm
Chloride ion<0.5 ppm0.1 ppt~5000 ppm
Suspended solids<5 ppb—~30,000 ppm
Table 2. Evaluation of possible accidents with different risks that could occur in an SCWO system.
Table 2. Evaluation of possible accidents with different risks that could occur in an SCWO system.
Accident LevelAccident Description
Level 1Failure of pumps
Failure of the three-phase separator
Level 2Overpressure
Over-heating
Failure of the heater
Opening of pressure relief valves
Level 3Burst or leakage of pipelines or equipment
Power outage
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Yang, J.; Xie, S.; Wang, S. Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge. Appl. Sci. 2024, 14, 5101. https://doi.org/10.3390/app14125101

AMA Style

Yang J, Xie S, Wang S. Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge. Applied Sciences. 2024; 14(12):5101. https://doi.org/10.3390/app14125101

Chicago/Turabian Style

Yang, Jianqiao, Shijing Xie, and Shuzhong Wang. 2024. "Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge" Applied Sciences 14, no. 12: 5101. https://doi.org/10.3390/app14125101

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