Control system design for a pressure-tube-type supercritical water-cooled nuclear reactor via a higher order sliding mode method

Nuclear power plants exhibit non-linear and time-variable dynamics.Therefore,designing a control system that sets the reactor power and forces it to follow the desired load is complicated.A supercritical water reactor(SCWR)is a fourth-generation conceptual reactor.In an SCWR,the non-linear dynamics of the reactor require a controller capable of control-ling the nonlinearities.In this study,a pressure-tube-type SCWR was controlled during reactor power maneuvering with a higher order sliding mode,and the reactor outgoing steam temperature and pressure were controlled simultaneously.In an SCWR,the temperature,pressure,and power must be maintained at a setpoint(desired value)during power maneuvering.Reactor point kinetics equations with three groups of delayed neutrons were used in the simulation.Higher-order and classic sliding mode controllers were separately manufactured to control the plant and were compared with the PI controllers speci-fied in previous studies.The controlled parameters were reactor power,steam temperature,and pressure.Notably,for these parameters,the PI controller had certain instabilities in the presence of disturbances.The classic sliding mode controller had a higher accuracy and stability;however its main drawback was the chattering phenomenon.HOSMC was highly accurate and stable and had a small computational cost.In reality,it followed the desired values without oscillations and chattering.

Supercritical water oxidation for the destruction of toxic organic wastewaters:A review

The destruction of toxic organic wastewaters from munitions demilitarization and complex industrial chemical clearly becomes an overwhelming problem if left to conventional treatment processes. Two options, incineration and supercritical water oxidation （SCWO）, exist for the complete destruction of toxic organic wastewaters. Incinerator has associated problems such as very high cost and public resentment; on the other hand, SCWO has proved to be a very promising method for the treatment of many different wastewaters with extremely efficient organic waste destruction 99.99% with none of the emissions associated with incineration. In this review, the concepts of SCWO, result and present perspectives of application, and industrial status of SCWO are critically examined and discussed.

Hydrogen generation from polyvinyl alcohol-contaminated wastewater by a process of supercritical water gasification

Gasification of polyvinyl alcohol （PVA）-contaminated wastewater in supercritical water （SCW） was investigated in a continuous flow reactor at 723-873 K, 20-36 MPa and residence time of 20-450 s. The gas and liquid products were analyzed by GC/TCD, and TOC analyzer. The main gas products were H2, CH4, CO and CO2. Pressure change had no significant influence on gasification efficiency. Higher temperature and longer residence time enhanced gasification efficiency, and lower temperature favored the production of H2. The effects of KOH catalyst on gas product composition were studied, and gasification efficiency were analyzed. The TOC removal efficiency （RTOC）, carbon gasification ratio （RCG） and hydrogen gasification ratio （RHG） were up to 96.00%, 95.92% and 126.40% at 873 K and 60 s, respectively, which suggests PVA can be completely gasified in SCW. The results indicate supercritical water gasification for hydrogen generation is a promising process for the treatment ofPVA wastewater.

Degradation mechanism of 2,4,6-trinitrotoluene in supercritical water oxidation

The 2,4,6-trinitrotoluene （TNT） is a potential carcinogens and TNT contaminated wastewater, which could not be effectively disposed with conventional treatments. The supercritical water oxidation （SCWO） to treat TNT contaminated wastewater was studied in this article, The TNT concentration in wastewater was measured by high-performance liquid chromatograph （HPLC） and the degraded intermediates were analyzed using GC-MS. The results showed that SCWO could degrade TNT efficiently in the presence of oxygen. The reaction temperature, pressure, residence time and oxygen excess were the main contributing factors in the process. The decomposition of TNT was accelerated as the temperature or residence time increased. At 550℃, 24 MPa, 120 s and oxygen excess 300%, TNT removal rate could exceed 99.9%. Partial oxidation occured in SCWO without oxygen. It was concluded that supercritical water was a good solvent and had excellent oxidation capability in the existence of oxygen. The main intermediates of TNT during SCWO included toluene, 1,3,5-trinitrobenzene, nitrophenol, naphthalene, fluorenone, dibutyl phthalate, alkanes and several dimers based on the intermediate analysis. Some side reactions, such as coupled reaction, hydrolysis reaction and isomerization reaction may take place simultaneously when TNT was oxidized by SCWO.

Hydrothermal decomposition of pentachlorophenol in subcritical and supercritical water with sodium hydroxide addition

Hydrothermal decomposition of pentachlorophenol （PCP, C6HC150）, as the probable human carcinogen, was investigated in a tubular reactor under subcritical and supercritical water with sodium hydroxide （NaOH） addition. The experiments were conducted at a temperature range of 30（0-420℃ and a fixed pressure of 25 MPa, with a residence time that ranged from 10 s to 70 s. Under the reaction conditions, the initial PCP concentrations were varied from 0.25 to 1.39 mmol/L, and the NaOH concentrations were varied from 2.5 to 25 times of the concentrations of PCP. The result of this study showed that PCP conversion in supercritical water was highly dependent on the reaction temperature, residence time, and NaOH concentration. PCP conversion in subcritical water is, however, only dependent on reaction temperature. NaOH concentration and residence times were found to have little effect on PCP conversion in subcritical condition. It was found that NaOH concentration affected the dechlorinations of PCP in the supercritical water. The intermediates detected were proposed to be tetrachlorophenol and trichlorophenol, respectively.

Decomposition kinetics of dimethyl methylphospate(chemical agent simulant) by supercritical water oxidation

Supercritical water oxidation （SCWO） has been drawing much attention due to effectively destroy a large variety of high-risk wastes resulting from munitions demilitarization and complex industrial chemical. An important design consideration in the development of supercritical water oxidation is the information of decomposition rate. In this paper, the decomposition rate of dimethyl methylphosphonate（DMMP）, which is similar to the nerve agent VX and GB（Sarin） in its structure, was investigated under SCWO conditions. The experiments were performed in an isothermal tubular reactor with a H2O2 as an oxidant. The reaction temperatures were ranged from 398 to 633℃ at a fixed pressure of 24 MPa. The conversion of DMMP was monitored by analyzing total organic carbon （TOC） on the liquid effluent samples. It is found that the oxidative decomposition of DMMP proceeded rapidly and a high TOC decomposition up to 99.99% was obtained within 11 s at 555℃. On the basis of data derived from experiments, a global kinetic equation for the decomposition of DMMP was developed. The model predictions agreed well with the experimental data.

Hydrogen production by glycerol reforming in supercritical water over Ni/MgO-ZrO_2 catalyst

Nano ZrO2 and MgO-ZrO2 were prepared by a self-assembly route and were employed as the support for Ni catalysts used in hydrogen production from glycerol reforming in supercritical water （SCW）. The reforming experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 600-800 ℃. The influences of process variables such as temperature, contact time, and water to glycerol ratio on hydrogen yield were investigated and the catalysts were charactered by ICP, BET, XRD and SEM. The results showed that high hydrogen yield was obtained from glycerol by reforming in supercritical water over the Ni/MgO-ZrO2 catalysts in a short contact time. The MgO in the catalyst showed significant promotion effect for hydrogen production likely due to the formation of the alkaline active site. Even when the glycerol feed concentration was up to 45 wt%, glycerol was completely gasified and transfered to the gas products containing hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At a diluted feed concentration of 5 wt%, near theoretical yield of 7 mole of H2/mol of glycerol could be obtained.

Effect of supercritical water on the stability and activity of alkaline carbonate catalysts in coal gasification

The stability and activity of alkaline carbonate catalysts in supercritical water coal gasification has been investigated using density functional theory method. Our calculations present that the adsorption of Na2CO3 on coal are more stable than that of K2CO3, but the stability of Na2CO3 is strongly reduced as the cluster gets larger. In supercritical water system, the dispersion and stability of Na2CO3 catalyst on coal support is strongly improved. During coal gasification process, Na2CO3 transforms with supercritical water into NaOH and NaHCO3, which is beneficial for hydrogen production. The transformation process has been studied via thermodynamics and kinetics ways. The selectively catalytic mechanism of NaOH and the intermediate form of sodium-based catalyst in water-gas shift reaction for higher hydrogen production has also been investigated. Furthermore, NaOH can transform back to Na2CO3 after catalyzing the water-gas shift reaction. Thus, the cooperative effects between supercritical water and Na2CO3 catalyst form a benignant circle which greatly enhances the reaction rate of coal gasification and promotes the production of hydrogen.

Decomposition of oil cleaning agents from nuclear power plants by supercritical water oxidation

Oil cleaning agents generated from nuclear power plants(NPPs)are radioactive organic liquid wastes.To date,because there are no satisfactory industrial treatment measures,these wastes can only be stored for a long time.In this work,the optimization for the supercritical water oxidation(SCWO)of the spent organic solvent was investigated.The main process parameters of DURSET(oil cleaning agent)SCWO,such as temperature,reaction time,and excess oxygen coefficient,were optimized using response surface methodology,and a quadratic polynomial model was obtained.The determination coefficient(R^(2))of the model is 0.9812,indicating that the model is reliable.The optimized process conditions were at 515 C,66 s,and an excess oxygen coefficient of 211%.Under these conditions,the chemical oxygen demand removal of organic matter could reach 99.5%.The temperature was found to be the main factor affecting the SCWO process.Ketones and benzene-based compounds may be the main intermediates in DURSET SCWO.This work provides basic data for the industrialization of the degradation of spent organic solvents from NPP using SCWO technology.

Effects of oxidants on the degradation of tributyl phosphate under supercritical water oxidation conditions

The effects of additional oxidants,such as NaNO_(3),Na_(2)S_(2)O_(3),KClO_(4),and K_(2)Cr_(2)O_(7),on the supercritical water oxidation(SCWO)of tributyl phosphate(TBP)were studied.The coupling of an ionic oxidant with SCWO can effectively enhance the oxidative degradation ability of the system,thus increasing its organic-matter-removal efficiency at a reduced reaction temperature.Moreover,the addition of NaNO_(3),KClO_(4),or K_(2)Cr_(2)O_(7)could improve this efficiency at a reaction temperature of 500℃compared with that of the original system at 550℃.Additionally,based on the conditions adopted in this study,the addition of either of these oxidants could reduce the final total organic carbon(TOC)of the effluent from~500 to<100 ppm.Concurrently,the ionic oxidants could effectively improve the processing capacity of the SCWO system to reduce the scale of the equipment,as well as the amount of produced wastewater.Compared with KClO_(4)and Na_(2)S_(2)O_(3),the addition of 10 mmol/L NaNO_(3)and K_(2)Cr_(2)O_(7)to the organic feed could increase the processing capacity of the system from 4 to 10%while maintaining the TOC removal at>99%.The effects of the ionic oxidants on the gas products,including CO_(2),CO,H_(2),and CH_(4),as well as other organic gases,have also been studied.Among these gas products,CO_(2)accounted for the main gas product with a proportion of more than half.At<500℃,temperature significantly affected the as products(CO,H_(2),CH_(4),and other organic gases).However,the gas product was mainly CO_(2)when the temperature was increased to≥500℃.This study initially revealed the enhancement effect of ionic oxidants on SCWO,which still requires further research.

Corrosion of candidate materials for supercritical water-cooled reactor

Supercritical water reactor(SCWR) was proposed as a GenerationⅣconcept for building large capacity nuclear power plants.Comparing with the present GenerationⅡandⅢlight water reactors,SCWR possesses great advantages of 10%higher efficiency,simpler system design,better sustainability,and so on. However,the selection of materials for fuel cladding and reactor internals of SCWR is facing a great challenge. Corrosion in supercritical steam is of the first important issue to be solved to meet the stringent requirement of the reactor internal components.Corrosion screening tests were conducted on candidate materials for nuclear fuel cladding and reactor internals of supercritical water reactor(SCWR) in static and re-circulating autoclave at the temperatures of 550,600 and 650℃,pressure of about 25 MPa,deaerated or saturated dissolved hydrogen(STP). Nickel base alloy type Hastelloy C276,austenitic stainless steels type 304NG,AL-6XN,HR3C.NF709 and SAVE 25,ferritic/martensitic(F/M) steel type P92,P122 and 410,and oxide dispersion strengthened steel MA 956,are tested.This paper presents corrosion rate,and focuses on the formation and breakdown of corrosion oxide film,and proposes the future trend for the development of SCWR internal structure materials.

Effect of supercritical water shell on cavitation bubble dynamics

Based on reported experimental data, a new model for single cavitation bubble dynamics is proposed considering a supercritical water （SCW） shell surrounding the bubble. Theoretical investigations show that the SCW shell apparently slows down the oscillation of the bubble and cools the gas temperature inside the collapsing bubble. Furthermore, the model is simplified to a Rayleigh-Plesset-like equation for a thin SCW shell. The dependence of the bubble dynamics on the thickness and density of the SCW shell is studied. The results show the bubble dynamics depends on the thickness but is insensitive to the density of the SCW shell. The thicker the SCW shell is, the smaller are the wall velocity and the gas temperature in the bubble. In the authors＇ opinion, the SCW shell works as a buffering agent. In collapsing, it is compressed to absorb a good deal of the work transformed into the bubble internal energy during bubble collapse so that it weakens the bubble oscillations.

Particle convective heat transfer near the wall in a supercritical water fluidized bed by single particle model coupled with CFD-DEM

Supercritical water fluidized bed(SCWFB)is a promising reactor to gasify biomass or coal.Its optimization design is closely related to wall-to-bed heat transfer,where particle convective heat transfer plays an important role.This paper evaluates the particle convective heat transfer coefficient(h_(pc))at the wall in SCWFB using the single particle model.The critical parameters in the single particle model which is difficult to get experimentally are obtained by the computational fluid dynamics-discrete element method(CFD-DEM).The contact statistics related to particle-to-wall heat transfer,such as contact number and contact distance,are also presented.The results show that particle residence time(τ),as the key parameter to evaluate h_(pc),is found to decrease with rising velocity,while increase with larger thermal boundary layer thickness.τfollows a gamma function initially adopted in the gas-solid fluidized bed,making it possible to evaluate h_(pc) in SCWFB by a simplified single particle model.The theoretical predicted h_(pc) tends to increase with rising thermal gradient thickness at a lower velocity(1.5 U_(mf)),while first decreases and then increases at higher velocity(1.75 and 2 U_(mf)).h_(pc) occupies 30%-57%of the overall wall-to-bed heat transfer coefficient for a particle diameter of 0.25 mm.The results are helpful to predict the overall wall-to-bed heat transfer coefficient in SCWFB combined with a reasonable fluid convective heat transfer model from a theoretical perspective.

Effects of physical properties of supercritical water on coarse graining of particle cluster

The coarse graining of particle cluster is of great significance to the study of a fluidized bed. The effects of variations in the physical properties of supercritical water on the coarse graining of particle cluster are investigated in this work. The drag coefficient distributions of the particle cluster are not influenced by the physical properties. However, the physical properties have effects on the values of drag coefficient. The effects of physical properties are weaker in the case of large particle concentrations. Furthermore, the physical properties lead to that the effect of particle cluster wake on the drag of downstream particles being significantly different from that of constant property flow. The variation trend of drag of coarse graining particle is consistent with that of isolated particle. The physical properties lead to significant differences in the values of drag. In this paper, the dominance of the effects of physical properties in a variety of cases is confirmed. Finally, a physical properties effect model is developed accordingly.

Investigation on a Supercritical Water Gasification System with CO_(2)as Transporting Medium

As a benign energy vector,hydrogen has been discussed for a long time.Supercritical water gasification was one of good ways to produce hydrogen.However,supercritical water gasification system with H_(2)O transporting was energy consuming in the process of heating due to the high specific heat of H_(2)O.A new supercritical water gasification system was established in this paper with supercritical CO_(2)as medium instead.Phenolic plastics were used as the sample transported by CO_(2).Production yields,energy flow and exergy flow of the system were collected and the influence of temperature,pressure,gasification concentration and transporting concentration was investigated.Mass flow of H_(2)O input into the reactor was 1000 kg/h.The typical condition was as follow:temperature 923.15 K,pressure 23 MPa,and the mass ratio of water,sample and transporting medium was 100:9:9.Yield of H_(2),CH4,CO and CO_(2)at this condition was 8.1 kg/h,39.6 kg/h,6.6 kg/h and 137.5 kg/h,respectively.Similar system with H_(2)O transporting was used to compare with the supercritical CO_(2)transporting system and proved that system with CO_(2)transporting could reduce the loss of both energy and exergy while the reduce of each gas production yield was less than 0.1 mol/mol.

ReaxFF-MD simulation investigation of the degradation pathway of phenol for hydrogen production by supercritical water gasification

Wastewater from the thermochemical conversion of coal and biomass contains a significant amount of phenolic structures compounds.The degradation of these phenolic compounds to hydrogen-rich gasses can prevent envi-ronmental pollution and save energy.Supercritical water(SCW)gasification of phenol is experimentally studied and a reactive force field molecular dynamics(ReaxFF-MD)simulation is conducted to investigate the catalytic mechanism of Ni/Al2 O3 in the phenol degradation.The experimental results indicate that Ni/Al2 O3 facilitates the conversion of phenol to 1-ethoxy butane via ring opening,which is a crucial step for complete gasification.The ReaxFF-MD simulation demonstrated that Ni facilitates the formation of H3 O free radicals and Ni-phenol inter-mediates.H3 O free radicals can be decomposed into H2 and OH free radicals.Both the generated OH free radical and Ni-phenol intermediate promote the ring-opening reaction of phenol.Ni promotes the direct decomposition of phenol into C1,C2,and C3 fragments,which is beneficial for further complete gasification.

Supercritical gasification for the treatment of o-cresol wastewater

The supercritical water gasification of phenolic wastewater without oxidant was performed to degrade pollutants and produce hydrogen-enriched gases. The simulated o-cresol wastewater was gasified at 440-650℃ and 27.6 MPa in a continuous Inconel 625 reactor with the residence time of 0.42-1.25 min. The influence of the reaction temperature, residence time, pressure, catalyst, oxidant and the pollutant concentration on the gasification efficiency was investigated. Higher temperature and longer residence time enhanced the o-cresol gasification. The TOC removal rate and hydrogen gasification rate were 90.6% and 194.6%, respectively, at the temperature of 650℃ and the residence time of 0.83 min. The product gas was mainly composed of H2, CO2, CFL and CO, among which the total molar percentage of H2 and CFL was higher than 50%. The gasification efficiency decreased with the pollutant concentration increasing. Both the catalyst and oxidant could accelerate the hydrocarbon gasification at a lower reaction temperature, in which the catalyst promoted H2 production and the oxidant enhanced CO2 generation. The intermediates of liquid effluents were analyzed and phenol was found to be the main composition. The results indicate that the supercritical gasification is a promising way for the treatment of hazardous organic wastewater.

Destruction of methylphosphonic acid in a supercritical water oxidation bench-scale double wall reactor

The destruction of methylphosphonic acid （MPA）, a final product by hydrolysis/neutralization of organophosphorus agents such as sarin and VX （O-ethyl S-[2-（diisopropylamino）ethyl] methylphosphonothionate）, was investigated in a a bench-scale, continuous concentric vertical double wall reactor under supercritical water oxidation condition. The experiments were conducted at a temperature range of 450–600°C and a fixed pressure of 25 MPa. Hydrogen peroxide was used as an oxidant. The destruction efficiency （DE） was monitored by analyzing total organic carbon （TOC） and MPA concentrations using ion chromatography on the liquid effluent samples. The results showed that the DE of MPA up to 99.999% was achieved at a reaction temperature of 600°C, oxygen concentration of 113% storichiometric requirement, and reactor residence time of 8 sec. On the basis of the data derived from experiments, a global kinetic rate equation for the DE of MPA and DE of TOC were developed by nonlinear regression analysis. The model predictions agreed well with the experimental data.

A New Cleaner Power Generation System Based on Self-Sustaining Supercritical Water Gasification of Coal

A new cleaner power generation system(IPGS) is proposed and investigated in this paper. Integrating combined cycle with supercritical water gasification of coal, the thermodynamic energy of the produced syngas is cascade utilized according to its temperature and pressure, both sensible and latent heat of the syngas can be recycled into the system, and thereby the net power efficiency can be about 6.4 percentage points higher than that of the traditional GE gasification based power plant(GEPP). The exergy analysis results show that the exergy efficiency of the proposed system reaches 52.45%, which is 13.94% higher than that of the GEPP, and the improvement in exergy efficiency of the proposed system mainly comes from the exergy destruction decline in the syngas energy recovery process, the condensation process and the syngas purification process. The syngas combustion process is the highest exergy destruction process with a value of 157.84 MW in the proposed system. Further performance improvement of the proposed system lies in the utilization process of syngas. Furthermore, system operation parameters have been examined on the coal mass fraction in the supercritical water gasifier(GF), the gasification temperature, and the gasification pressure. The parametric analysis shows that changes in coal concentration in the GF exert more influence on the exergy efficiency of the system compared with the other two parameters.

Supercritical water oxidation of polyvinyl alcohol and desizing wastewater: Influence of NaOH on the organic decomposition

Polyvinyl alcohol is a refractory compound widely used in industry. Here we report supercritical water oxidation of polyvinyl alcohol solution and desizing wastewater with and without sodium hydroxide addition. However, it is difficult to implement complete degradation of organics even though polyvinyl alcohol can readily crack under supercritical water treatment. Sodium hydroxide had a significant catalytic effect during the supercritical water oxidation of polyvinyl alcohol. It appears that the OH ion participated in the C-C bond cleavage of polyvinyl alcohol molecules, the CO2-capture reaction and the neutralization of intermediate organic acids, promoting the overall reactions moving in the forward direction. Acetaldehyde was a typical intermediate product during reaction. For supercritical water oxidation of desizing wastewater, a high destruction rate （98.25%） based on total organic carbon was achieved. In addition, cases where initial wastewater was alkaline were favorable for supercritical water oxidation treatment, but salt precipitation and blockage issues arising during the process need to be taken into account seriously.