Flue Gas Residence Time Research Articles

Improving the performance of a condensing heat exchanger for biomass combustion at household scale

A computational CFD model representing the behavior of a condensing heat exchanger used in domestic heating and hot water applications has been presented and validated, obtaining a flow output gas temperature error of 7%. An analysis of the heat exchanger internal aerodynamic performance revealed the need for geometry enhancements which involved increasing the number of combustion gas passages, reducing the cannon diameter, and incorporating turbulence generators.The geometric modifications produced a significant increase in variables of interest for the performance of the equipment when compared to the baseline case. The improvement is quantified as the 68% increase in turbulent intensity, 57% increase in velocity, doubling of flue gas residence time and 96% increase in total heat transfer rate.The design that contains turbulence generators presents the best performance, obtaining the highest heat transfer rate equal to 3.5 kW, which produces a decrease in the flue gas temperature equal to 62% and an increase in the water outlet temperature equal to 53% compared to the base case. In contrast, the prototype without turbulence generators showed a relatively lower performance in terms of heat transfer, but with a 9 times lower pressure drop.

Study on the evaporation performance of concentrated desulfurization wastewater and its products analysis

Concentrated desulfurization wastewater evaporation technology exhibited significant superiority in desulfurization wastewater zero discharge. Nevertheless, changes of physical and chemical properties of the concentrated wastewater shifts evaporation mode during wastewater evaporation, which may cause difficulty in disposing of evaporation products. In this work, the rotary spray drying pilot test experimental platform was utilized to investigate the evaporation performance of concentrated desulfurization wastewater. The results indicate that, while the evaporation rate of concentrated wastewater is lower than that of non-concentrated wastewater, its lower free water volume resulting in the improved evaporation performance. Increasing the inlet flue gas temperature, gas-liquid ratio, and prolonging the residence time of flue gas can promote droplet evaporation. Besides, the evaporation products were analyzed by various characterization techniques. The elemental composition and particle size of concentrated wastewater evaporation products change. At a moisture content of 2 %, the Flowability Function is 3.57 (2 < FF < 4), indicating a cohesive nature and poorer flowability compared to the non-concentrated wastewater.

Mathematical modeling of a 30 MW biomass-fired grate boiler: A reliable baseline model taking fuel-bed structure into account

Grate boilers are widely applied in biomass combustion for heat and energy generation. However, grate-firing systems often suffer from low efficiency and high emission. Computational fluid dynamic (CFD) becomes increasingly popular as a flexible method to obtain the detailed combustion behaviors in the furnace, and to optimize the performance of existing grate boilers. This paper presents efforts toward a reliable baseline CFD model for an industrial biomass-fired grate boiler, where the structure of the fuel-bed was sufficiently considered. The combustion performances in the furnace were investigated experimentally and numerically by coupling the fuel bed and freeboard, to clarify the impact of bed structure on the simulation accuracy. Results show that when the bed structure has been taken into account, the predictions are in better agreement with the various measurements, as the deviations between them are all within 10 %. Moreover, the flame center drops, the total heat transferred to the water-cooled walls increases, and the residence time of flue gas is greatly prolonged from 2.76 s to 3.90 s, resulting in a distinct difference of the temperature and species patterns in the freeboard. The simulations will provide valuable theoretical guidance for the accurate modeling of grate-fired boiler.

SO3/sulfuric acid mist removal in simulated flue gas: Multi-factor study based on two-film theory mass transfer process

SO3/sulfuric acid mist is a strongly irritating pollutant presented in small quantities in flue gas, which poses a major threat to the power plants operation and atmospheric environment. In this study, SO3/sulfuric acid mist at different positions and after different spraying time of the small desulfurization tower were extracted and absorbed by isopropanol solution, and the SO42- concentration was detected by barium-thorium titration. Then the SO3/sulfuric acid mist mass transfer model in desulfurization process was constructed on the basis of two-film theory to investigate the effects of different tower structures, spraying time and operating conditions on SO3/sulfuric acid mist removal in desulfurization tower. The results showed that the tower structures had the greatest influence on SO3/sulfuric acid mist removal, and the highest SO3/sulfuric acid mist removal efficiency was achieved after the spraying time of 0–20 min and at the flue gas distribution plate. Moreover, according to the results of SO3/sulfuric acid mist mass transfer model, SO3 removing process was not controlled by solid dissolution instead of by gas–liquid film turning to gas film with flue gas residence time running in the tower. The model results also showed that SO3 removing process was controlled by gas film for a very short period then turning to gas–liquid film immediately with spraying time accumulating in a specific tower position. Besides, it showed the increase of NH3 concentration and spraying flow rate promoted the SO3/sulfuric acid mist removal, and the increase of SO2 concentration and slurry temperature inhibited the SO3/sulfuric acid mist removal.

The removal of NO from flue gas by Fe/NaOH/H2O2 system

NO emitted from coal combustion was harmful to the environment. For most current wet denitrification technologies, obtaining the desired removal efficiency was always dependent on the contact time of NO with the reaction solution. In this paper, a novel Fe/NaOH/H2O2 technology was proposed to remove NO, which could obtain 90% NO removal efficiency when the flue gas residence time in solution was 3 s. The mechanism of H2O2 decomposition and NO removal were analyzed. Results demonstrated that Fe and NaOH together catalyzed H2O2 to produce radicals, and ⋅O2− radicals took the main responsibility in the process of NO removal. NO3− and NO2− were the final forms of NO. In addition, the effects of several common experimental parameters were investigated on the NO removal performance. The increase of H2O2 concentration, Fe concentration, solution pH and reaction temperature all had a positive effect on NO removal, but this process was also accompanied by an intensification of the useless decomposition of H2O2 (consumption of H2O2 in the form of O2). The addition of SO2 to the flue gas had a slight effect on the NO removal and the useless decomposition of H2O2.

Numerical study on the distribution of flue gas residence time in the desulfurization and denitrification system by the optimization of the model

Abstract During the operation of the CSCR activated carbon desulfurization and denitrification flue gas purification system, due to the unreasonable design of flue gas pipe diameter and the uneven distribution of activated carbon porosity, the uniformity of flue gas distribution is prone to problems, which affects flue gas removal efficiency and increases operating costs. In this article, the Fluent software was used to establish a three-dimensional numerical model of the flue gas purification system, and the flow characteristics of the flue gas were studied by continuously optimizing the structure of the system model. The effects of different pipe diameters, the accumulation method of activated carbon in the desulfurization and denitration module and the distribution of porosity on the flue gas are analyzed, and the effects on the average residence time and variance of flue gas were further analyzed. The results show that the average residence time, the variance, the dead volume fraction, and well-mixed volume decreased, while the dispersed plug volume fraction increased for a module after optimization. The average residence time and the dispersed plug volume fraction decreased. While the variance, the dead volume fraction, and well-mixed volume fraction increased for the system after optimization.

The removal of NO from flue gas by NaOH-catalyzed H2O2 system: Mechanism exploration and primary experiment

Currently, most advanced oxidation denitrification technologies require long flue gas residence time to obtain ideal NO removal efficiency. The NaOH-catalyzed H2O2 system proposed in this paper can obtain 98% NO removal efficiency under the condition of flue gas residence time of 3 s. The mechanism of NO removal and H2O2 decomposition to O2 were proposed. It was confirmed with ESR (Electron-spin-resonance), inhibitor experiments and UV-Vis spectrophotometer that the main group in the reaction process was·O2- radicals, which reacted with NO to form ONOO-, and ONOO- would be gradually transformed into NO3- and NO2- in the air. The effect of some primary factors on the NO removal efficiency and the percentage of H2O2 decomposition to O2 were also investigated. The increase of initial pH has a positive effect on NO removal, while the promotion of NO removal by increasing H2O2 concentration and reaction temperature is limited and the increase of NO has a negative effect on NO removal. Initial pH has a dual impact on the percentage of H2O2 decomposition to O2, H2O2 concentration and reaction temperature promote the decomposition of H2O2 to O2, while NO concentration has an inhibiting effect on it.

Investigation of a novel combustion stabilization mechanism and combustion characteristics of a multi-nozzle array model combustor

Short and uniformly dispersed flames in multi-nozzle array combustors allow for shorter flue gas residence time, resulting in less NOx emissions at high combustor outlet temperature. A new combustion stabilization mechanism based on high-speed jet induced flue gas recirculation has been utilized in an array model combustor. The stabilization mechanism allows for stable combustion at higher nozzle outlet velocity. The flame stabilization mechanism and combustion characteristics for the model combustor fueled methane have been investigated by using the combination of experiments and numerical simulations. The parameters studied include nozzle outlet velocity, inlet air temperature, and heat loss ratio. The combustion characteristics include NOx/CO emissions and reaction zone distribution. The results show that the flame stabilization mechanism is a result of the combined effects of the inner and outer recirculation zone. Under atmospheric conditions within the adiabatic temperature range from 1500 K to 2100 K, the NOx emissions are less than 20 ppm@15%O2 and the CO emissions are less than 30 ppm@15%O2. As the heat loss ratio decreases, the NOx emissions for the model combustor increase. When the nozzle outlet velocity increases, the lean blowout (LBO) for the model combustor increases. With increasing inlet air temperature, the equivalent ratio range of combustion stability expands. The downstream of the reaction zone is mostly affected by the nozzle outlet velocity and inlet air temperature, while the root of the reaction zone is essentially unaffected due to the role of inner recirculation zone. The unique stabilization mechanism of the multi-nozzle array combustion is beneficial for fuel flexibility.

Numerical simulation analysis on the characteristics of municipal domestic waste with high calorific value and sludge co-combustion

The combustion characteristics of a high calorific value domestic waste blend with municipal sludge of 350 t/d, 750 t/d, and 900 t/d in three kinds of mainstream mechanical waste incinerators in Guangdong were studied by numerical simulation method, and the model was verified with the operating data. Based on the “3 T” principle of incinerator, the combustion characteristics of mixed fuel with high calorific value municipal solid waste and sludge in different incinerators were analyzed. The results showed that when the maximum blending ratios of sludge in the 350 t/d, 750 t/d, and 900 t/d waste incinerators were 13%, 17%, and 10%, respectively, and the residence times of flue gas in the temperature region above 1123 K were 4.18 s, 3.68 s, and 6.36 s, respectively. The 750 t/d waste incinerator adopts a spiral secondary air injection mode, which is beneficial to the realization of “3 T” principle. The secondary air of the 350 t/d waste incinerator adopts the hedge injection mode, the first flue velocity is larger, and the flue gas residence time is shorter. 900 t/d waste incinerator combined with spiral and hedge injection, the temperature at the exit of one flue decreased by 61.12 K and 132.62 K compared to 0% working condition for 10% and 17% sludge blending, respectively, and the oxygen content increased from 2.97% to 3.91% and 5.63%. The appropriate primary and secondary air ratio and effective secondary air configuration can adjust the incinerator to be in the best operating condition.

Experimental study on the removal of PM2.5 using modified carbon steel collectors with hydrophilic properties in wet ESPs

Along with the increasingly stringent dust emission concentration standards for the thermal power plants and the challenge of market competition, wet electrostatic precipitators (ESPs) have been widely used as terminal equipment for the flue gas treatment. The wet ESPs pilot test platform using the modified carbon steel collectors with hydrophilic properties was established in this paper and the experimental research on PM2.5 removal characteristics were carried out. The mechanism of the particles removal efficiency enhancement by water film on the surface of modified collectors was studied. The effect of gas temperature, residence time, working voltage, inlet concentration and flushing water flowrate on the particles removal efficiency was investigated. The results indicated that the fibrous layer over the surface of the modified collectors could reduce the effect of the airflow recoil and the electromigration resistance of particles. The surface of the modified collectors could also maintain a uniform and stable water film with a lower amount of water consumption. The water film could restrain the back corona and the re-entrainment of dust. And the evaporation of water film increased the humidity of the flue gas, meanwhile, the particles charge capacity and electromigration velocity increased. Those all could improve the particles removal efficiency. The particles removal efficiency increased with the increase of the flue gas residence time and the applied voltage, but the particles removal efficiency improved less with the increase of the particles inlet concentration and flushing water flow rate after maintaining uniform and stable water film. The modified rigid collector provided high removal efficiency for particles with diameters of 0.04～0.48 μm under lower energy and water consumption. Implications: The wet electrostatic precipitators (ESPs) have been widely used as terminal equipment of the flue gas treatment to purify the flue gas deeply. The surface of the modified carbon steel collectors with hydrophilic properties could maintain a uniform and stable water film with a lower amount of water consumption. The evaporation of water film improved the particle charge capacity and electromigration velocity. The wet ESPs using the modified rigid collector exhibited significantly higher removal efficiency of particles than using the conventional rigid collector especially to the particles with diameter of 0.04～0.48 μm.

Experimental Study on Removal of SO3 from Flue Gas by Na2CO3

The alkaline absorbent spraying technology is an efficient method for accurately removing SO3 in flue gas. By studying the characteristics of the technology, the reaction mechanism, the migration rule of SO3, and the experimental platform by using the real flue gas as the source of the alkaline absorbent jet removal of flue gas SO3. The results of the study show that the efficiency of the alkaline absorbent to remove SO3 is related to n(Na2CO3):n(SO3), flue gas temperature, residence time and other factors; under the test conditions, the flue gas temperature is 320°C, n(Na2CO3): n(SO3) is 1:4, the concentration of Na2CO3 solution after gasification is 52.6 mg/m3, and the concentration of soot is 4680 mg/m3 under standard conditions, which can ensure the removal efficiency of SO3 is above 85%; The effect of the catalyst is small and can be ignored. The above results can provide a reference for the domestic coal-fired power plants to carry out the research and application of alkaline absorbent to remove flue gas SO3 technology.

Simultaneous removal of SO2, NO and Hg0 using an enhanced gas phase UV-AOP method

The core for simultaneous removal of SO2, NO and Hg0 is the oxidation of NO and Hg0. Radical induced oxidation of NO and Hg0 is considered to be the most efficient method. We develop a novel gas phase advanced oxidation process (AOP) of UV-Heat/H2O2-NaClO2 to simultaneously remove SO2, NO and Hg0 due to a great synergism between H2O2 and NaClO2 under thermal and ultraviolet (UV) co-catalysis. The results indicated that the SO2 removal was always good, while the removal of NO and Hg0 was affected by NaClO2 and UV. Higher catalytic temperature and longer flue gas residence time favored the removal of NO and Hg0. The presence of SO2 and NO facilitated Hg0 removal. Kinetics analyses were conducted to provide the reaction rate of removal of NO and Hg0 under different conditions. X-ray photoelectron spectroscopy (XPS) revealed the product composition as Cl−, Hg2+, NO3− and SO42−. Electron spin resonance (ESR) tests confirmed the generation of HO. Cost analyses demonstrated the better cost performance of the proposed method compared to SCR-ACI combined method. HO and ClO2 were proved to be the main oxidant. The reaction mechanism for removal of NO and Hg0 by using UV-Heat/H2O2-NaClO2 were proposed finally.

Using excess O3 to facilitate the NO2 absorption in a sulfite solution: Process conditions and mechanism

Excess O3 is invariably used to ensure the effective oxidation of NO in the industrial simultaneous desulfurization and denitration through combined ozone oxidation and absorbent scrubbing. O3 is difficult to fully react with NO considering the short residence time of flue gas, remaining O3 transport into the subsequent absorption device, and even its escape into the atmosphere. To study the influence of excess O3 on NO2 absorption in sulfite solution, we simulated an integrative process of pre-oxidation and post-absorption. NO2 removal efficiency was improved from 36.1% to 66.5% as the mole ratio of O3/NO increased from 1.0 to 1.6. Additionally, the effects of temperature, pH, and reactant concentration with the remaining O3 on NO2 removal were evaluated. After 5 h of steady absorption, the concentration of total N species in the adsorbent was 82.8 mmol/L at a O3/NO mole ratio = 1.6. However, this concentration was only 51.4 mmol/L at a O3/NO mole ratio = 1.0, indicating that more NO2 was absorbed, and the gas-liquid transfer of NO2 was enhanced by the excess O3 via oxidizing NO2− into NO3−. This work contributes to re-utilizing remaining O3 and reducing O3 escape in simultaneous desulfurization and denitration technology.

Parameter design of rotary kiln incinerator and application analysis in engineering cases

In this paper, according to the type and scale of hazardous waste incineration, process selection, design of rotary kiln incinerator, and the engineering principle and performance characteristics of rotary kiln incinerator system are analysed, and the proposed method is verified by practical engineering examples. The results showed that the temperature of rotary kiln is controlled at about 850 °C, the residence time of hazardous waste in rotary kiln is about 60min, the rotating speed is 0.88r/min, and the inclination angle is 1.75. The temperature of the second combustion chamber is more than 1100 °C, and the residence time of flue gas in the second combustion chamber is more than 2S. After continuous incineration, the reduction rate of slag ignition is less than 5%, the removal rate of fly ash is more than 99.99%, and the flue gas is discharged up to the standard.

Removal of gaseous elemental mercury using thermally catalytic chlorite-persulfate complex

We developed a novel oxidation method, thermally catalytic chlorite-persulfate (NaClO2-Na2S2O8, shorten as CP), to remove gaseous Hg0. Chlorite and persulfate exhibited an outstanding performance in Hg0 removal as compared to using chlorite or persulfate alone. The cost for using CP is 3–69 USD/lb-Hg0 which is far lower than using Na2S2O8 (1739–2552 USD/lb-Hg0) and NaClO2 (37–1299 USD/lb-Hg0). Kinetics analysis indicated that the rate constant of the reaction between CP and Hg0 is 5250 times and 11.7 times higher than that of using Na2S2O8 and NaClO2 alone. The impacts of CP concentration, thermal catalytic temperature, flue gas residence time and presence of other gases in the flue gas on the Hg0 removal were evaluated. SO2 and NO were found to be the inhibitor and promoter, respectively, for Hg0 removal. Mercury distribution under different atmospheric conditions was determined by using atomic fluorescence spectrometry (AFS). The active oxidizing species produced in the thermal catalysis of CP were determined to be ClO2, SO4− and HO using ultraviolet–visible spectrophotometry (UV–Vis) and electron spin resonance (ESR). HgCl2 and HgSO4 were the main mercury forms in the byproduct according to the X-ray photoelectron spectroscopy (XPS) result. The mechanism of Hg0 removal with using CP was proposed.

Combustion Characteristics of Plant Chemical Polyol Waste Liquor in a Pilot Water-Cooled Incinerator

In the production of plant chemical alcohol, a large amount of salty high-concentration organic waste liquid is by-produced, in which a large amount of high-value sodium tungstate and chemical heat energy are contained. Through the design of the pilot water-cooled incinerator, the operating parameters and the characteristics of alkali ash deposition of the waste incinerator were tested. The pilot incinerator is a natural-cycle steam boiler, has a full water-cooled II-type structure, with liquid slag discharge, and the furnace design has selected reasonable volumetric heat load and flue gas residence time. Combustion distribution is the core parameter affecting the combustion of waste liquid. Studies have shown that increasing the primary air rate and the excess air coefficient are beneficial to improve the combustion performance of waste liquid, increase steam production, and reduce CO and SO2 emissions in flue gas, but also increase the concentration of NO. When the excess air ratio was 1.2, and the primary air ratio was 0.65, the NO concentration in the tail flue gas was less than 10 ppm, and the SO2 concentration was zero. The TG-DTA method measured that when the alkali ash melting temperature was less than 587 °C, the alkali ash had strong adhesion in the high temperature zone of the incinerator and less deposition in the low temperature region of the tail. The alkali ash deposition rate and relative deposition rate were used to quantitatively study the characteristic of alkali ash deposition.

Dry Sorbent Circulating Reactor System for Removing Acid Gas Pollutants Emissions from Power and Incineration Utilities

Dry type acid gas pollutants removal processes offer the significant advantages of low capital and operating costs when compared to wet type acid gas removal processes. They hold great potential for the economical reduction of SO2 and HCl emissions from power and incineration utilities that use high-sulfur coal and high halide base acids wastes. One of the project’s major goals was the development of dry, calcium-based sorption processes for removing sulfur dioxide and hydrogen chloride from the combustion gases and incineration gases produced by high-sulfur coal and high halide base acids wastes. Dry sorbent circulating reactor system for flue gas cleaning highlights a number of experimental research findings that have had a significant and lasting impact in term of scientific understanding. For example, the experimental investigation in demonstration test unit SO2 and HCl capture by dry sorbent obtained removal efficiency more than 99%, thereby revealing the well fluid mixing with sorbent and flue gas and longer residence time in the reactor. The average HCl concentration at the inlet of the reactor maintained at 878 ppm. The HCl removal efficiency maintained at over 99.9%, and the HCl emission concentration maintained at below 5 ppm under the conditions that the flow amount of the flue gas was 40,000 Nm3/hr, the SR was below 2.0, and the pressure loss at the baghouse was 150 mmH2O. We also identified a number of important areas for future research, including reaction mechanisms, sorbent material, transport effects, simulation of particle fluid dynamics and efficient system development.

An evaluation of effects of operational parameters on NOx emissions through detailed chemical kinetics simulations

Abstract NOx emissions from biomass and Municipal Solid Waste (MSW) combustion is a great concern. Technologies for reducing NOx emissions based on flue gas cleaning using e.g. Selective Non-Catalytic Reduction (SNCR) or Selective Catalytic Reduction (SCR) are well known. However, limiting NOx emissions by primary measures is an attractive opportunity, avoiding or limiting the use of costly secondary measures. Many factors influence the NOx emission level from biomass and MSW combustion plants, like fuel-N content and speciation, and operating parameters as air distribution and staging, flue gas recirculation, temperature and residence time. In this work a recent detailed chemical kinetics mechanism is used to investigate the NOx reduction potential by staged air combustion at different operational conditions, using two ideal plug flow reactors connected in series. This enables revealing a kinetics limited reduction potential, as a guideline for further detailed studies at targeted conditions using stochastic reactor networks or Computational Fluid Dynamics (CFD). The results show, for demolition wood as fuel, a large NOx reduction potential, as high as above 95% total fixed nitrogen reduction at theoretically optimum conditions. Through reaction path analyses the main reaction pathways and reactions are identified. Comparison with experimental results from grate combustion units is made. Finally, a comparison with earlier and less extensive detailed chemical kinetics mechanisms is made, to check for significant differences and assess the reasons for those. The final goal would be to arrive at computationally effective reduced chemical kinetics mechanisms that could be applied in CFD simulations of real biomass and MSW combustion plants, to arrive at improved operational conditions or improved design.

Rate of hexabromocyclododecane decomposition and production of brominated polycyclic aromatic hydrocarbons during combustion in a pilot-scale incinerator

Here, we examined the incineration of extruded polystyrene containing hexabromocyclododecane (HBCD) in a pilot-scale incinerator under various combustion temperatures (800–950°C) and flue gas residence times (2–8sec). Rates of HBCD decomposition ranged from 99.996% (800°C, 2sec) to 99.9999% (950°C, 8sec); the decomposition of HBCD, except during the initial stage of combustion (flue gas residence time<2sec), followed a pseudo-first-order kinetics model. An Arrhenius plot revealed that the activation energy and frequency factor of the decomposition of HBCD by combustion were 14.2kJ/mol and 1.69sec−1, respectively. During combustion, 11 brominated polycyclic aromatic hydrocarbons (BrPAHs) were detected as unintentional by-products. Of the 11 BrPAHs detected, 2-bromoanthracene and 1-bromopyrene were detected at the highest concentrations. The mutagenic and carcinogenic BrPAHs 1,5-dibromoanthracene and 1-bromopyrene were most frequently detected in the flue gases analyzed. The total concentration of BrPAHs exponentially increased (range, 87.8–2,040,000ng/m3) with increasing flue gas residence time. Results from a qualitative analysis using gas chromatography/high-resolution mass spectrometry suggest that bromofluorene and bromopyrene (or fluoranthene) congeners were also produced during the combustion.

Technology development for the reduction of NOx in flue gas from a burner-type vaporizer and its application

We developed a modified process of a submerged combustion vaporizer (SMV) to remove nitric oxides (NOx) efficiently from flue gas of the SMV at liquefied natural gas (LNG) terminals. For this, excess oxygen is injected into exhaust gas that contains NOx from SMV burner. Then, the mixed gas spreads into a hydrogen peroxide solution or water bath. We initially performed experiments of the modified system to estimate the effect of various process variables (temperature, excess O2 concentration, pH of water, residence time of flue gas in water tank, and H2O2 concentration) on NOx conversion, and developed a mathematical model of the system based on the experiment results. Lastly, we confirmed higher performance of the modified system and validated the feasibility for its field application.