Wet Limestone Flue Gas Desulfurization Research Articles

A multi-objective optimization strategy of steam power system to achieve standard emission and optimal economic by NSGA-Ⅱ

In this work, the models of desulfurization and denitrification are added to solve the problem of SO2 and NOX standardized discharge. The design and optimization strategy of steam power system (SPS) considering contaminant emissions reduction technology is proposed to achieve the trade-off between economic and environmental goals. Detailed superstructure networks of desulfurization based on wet limestone flue gas desulfurization and denitrification based on selective catalytic reduction were established and embedded in the SPS model. Then, based on this combined superstructure model, a mathematical formulation of multiple objective mixed integer nonlinear programming describing the SPS coupled with desulfurization and denitrification was established. The steam flow rate, outlet enthalpy, the consumption of the turbine power of the direct drive equipment and the electricity generated by the turbine, the flow rate and efficiency of desulfurization and denitrification are chosen as the optimization variables. The operating conditions and equipment parameters of the global system are optimized. Finally, the second-generation non-dominated sorting genetic algorithm (NSGA-II) was applied to obtain the Pareto optimization curve, exploring trade-offs between economic and environmental goals. Two case studies are used to assess the applicability and performance of the optimization formulation and solution algorithm.

Influence of Typical Desulfurization Process on Flue Gas Particulate Matter of Coal-fired Boilers

As flue gas desulfurization (FGD) was one of the most important purification processes of coal-fired boilers, we selected four boilers, which were equipped with wet limestone, furnace calcium injection, ammonia-based, and double-alkali FGDs, to research the influence of FGDs on the flue particulate matter (PM). The flue PM before and after the FGD were sampled using laboratory resuspension and dilution tunnel sampling methods, respectively, and the PM was analyzed for its chemical composition (i.e., ions, elements, and carbon). The results showed that the types of desulfurizers could influence the composition of the flue PM. After passing through the wet limestone, ammonia-based, and double-alkali FGDs, the proportion of Ca, NH4+, and Na in PM2.5 increased from 5.1% to 24.8%, from 0.8% to 7.3%, and from 0.9% to 1.7%, respectively. The influence of wet and dry FGDs on the flue PM were different. The fraction of ions in the PM emitted from the wet FGD were higher than those from the dry FGD. The proportion of SO42- in the flue PM2.5 increased from 2.0% and 6.7% to 9.6% and 11.9% using the wet limestone and ammonia-based FGDs, respectively, and Cl- increased from 0.4% and 1.2% to 3.8% and 5.2%. In addition, the amount of heavy metals (e.g., Cr, Pb, Cu, Ti, and Mn) in PM2.5 declined after the wet FGDs. The PM2.5 emitted from the dry FGD boiler was richer in crustal elements, such as Al, Si, and Fe, than that from the wet FGDs. The wet FGDs also effected the carbonaceous components of the flue PM. After passing through the wet limestone and ammonia-based FGDs, the proportion of elemental carbon in the flue PM2.5 decreased from 6.1% to 0.9% and from 3.6% to 0.7% respectively, but the organic carbon content did not decrease.

Effect of ammonia and ammonium compounds on wet-limestone flue gas desulfurization process from a coal-based power plant – Preliminary industrial scale study

The paper encompasses a review and presents the influence of ammonia and ammonium ions on wet flue gas desulfurization (WFGD) absorber based on results of an preliminary industrial scale study. Subject of the study was a WFGD with total maximum flue gas flow capacity equal to 1,500,000 Nm3/h, fed from four hard-coal fueled boilers equipped with SNCR based on urea injection. In this study, effects of SNCR operation and ammonia slip on the absorption slurry pH and NH4+ concentration are considered. It was shown, that an increase in ammonia concentration present in the flue gas upstream the WFGD increases the pH of absorption slurry, a critical parameter in the control of WFGD operation, limestone dissolution and gypsum formation. Moreover, analysis of particulate matter upstream and downstream the absorber shows, that the WFGD as a source of particulate matter increases the content of ammonium species in the PM due to the entrainment and evaporation by 3,9 times, as average. Results show that a SNCR system combined with a WFGD can achieve higher concentrations of NH4+ ions in the absorption slurry (up to 768 mg/L), gypsum slurry (up to 858 mg/L) and raw wastewater (up to 891 mg/L) compared to the more commonly used SCR. Results indicate, that the wastewater must be additionally treated prior the discharge to the environment.

Effect of additives on mercury partitioning in wet-limestone flue-gas desulfurization

AbstractThe wet-flue-gas desulfurization (FGD) process plays an important role in removing water-soluble flue-gas components such as sulphur dioxide (SO2) and oxidized mercury compounds. Under the reducing environment of the FGD, there is the possibility of re-emission of the already absorbed mercury (Hg) to the gas phase, which may be diminished by the utilization of specific additives. In this study, the effect of two different additives on Hg re-emission from the aqueous phase and Hg partitioning in gypsum and filtrate of a lab-scale wet-limestone FGD is investigated. Furthermore, the behaviour of additives in the presence of different halides is studied. The studied additives are TMT 15® as a sulphidic precipitating agent, which forms non-soluble mercury compounds, and activated lignite (AL) as a carbon-based sorbent, which adsorbs Hg compounds from the aqueous phase. TMT 15® has no significant effect on SO2 absorption; on the other hand, addition of AL improves the SO2-removal efficiency by up to 30%. Using both additives, Hg re-emission is suppressed in all the experimented cases except for AL in the absence of halides, in which Hg re-emission shows no change. Thus, the need to form nucleophilic oxidized mercury compounds in the slurry for the adsorption of oxidized mercury on AL can be concluded. Usage of both additives improves Hg retention in the slurry to different extents. It is shown that, for the additive-free slurries, the Hg-adsorption capacity of the solid fraction of the slurry is the limiting parameter. Moreover, the utilization of both additives results in a significant increase in the Hg concentration of solid fraction. The correlation between redox potential and partitioning of Hg in the slurry is presented by comparing the change in the redox potential of slurries when additives are used.

Influence of ammonia on wet-limestone flue gas desulfurization process from coal-based power plant

The paper presents data concerning the influence of ammonia on wet flue gas desulfurization (WFGD) absorber. Paper presents preliminary results of an industrial scale study on WFGD absorber, that collects flue gases from four boilers with total flue gas flow capacity of the WFGD equal to 1 500 000 Nm3/h. Each boiler is equipped with selective non-catalytic reduction (SNCR) with urea injection. It was shown, that ammonia present in the flue gas upstream the WFGD increases the pH of absorption slurry, impacting the desulphurization process. In addition, analysis of particulate matter upstream and downstream the absorber shows an increase of the concentration of ammonium species in the particulate matter about 14 times, as average. Moreover, a non-optimized denitrification process can cause a high NH4+ concentration in the absorption slurry (up to 768 mg/L) and raw wastewater (up to 891 mg/L).

Use of the Eichhornia crassipes as possible biosorbent for the removal of heavy metals from wet limestone flue gas desulfurization plant (WLFGD) wastewater

Use of the Eichhornia crassipes as possible biosorbent for the removal of heavy metals from wet limestone flue gas desulfurization plant (WLFGD) wastewater

Efficient utilisation of flue gas desulfurization gypsum as a potential material for fluoride removal

This study introduces the use of a waste by-product from wet limestone flue gas desulfurization as a potential material for fluoride removal. Systematic laboratory-scale experiments were tested to identify the fluoride removal performance and determine the underlying mechanism. Flue gas desulfurization (FGD) gypsum removes 93.31% of fluoride from 109 mg/L to 7.3 mg/L. Fluoride can be efficiently removed at the optimum pH range of 5–11. Kinetics analysis indicates that the theoretical fluoride capacity at 1 g/L FGD gypsum is 96.9 mg/g. Equilibrium speciation analysis indicates that the decrease of system pH to lower than 5 is unsuitable for the formation of calcium fluoride, and the increase of system pH to higher than 11 opposes calcium release from FGD gypsum. Thermodynamic analysis confirms the feasibility of converting calcium sulphate into calcium fluoride at pH > 5. FGD gypsum and precipitates were characterized to describe their surface morphology, elemental composition and crystalline phase. Results indicate that FGD gypsum removes fluoride through the combination of calcium with fluoride to generate calcium fluoride.

Utilization of coal fly ash and bottom ash as solid sorbents for sulfur dioxide reduction from coal fired power plant: Life cycle assessment and applications

Feasibility of sulfur dioxide (SO2) reduction from a coal fired power plant using fly ash and bottom ash as solid sorbents is evaluated. The study integrates inventory data from experiments, process simulation, published studies, and real plant operation for comprehensive life cycle assessment (LCA). The experimental results exhibit the capability of free lime contained fly ash and bottom ash to react with SO2 under the least efficient conditions. The scenario-based LCA results show that fly ash/bottom ash utilization can lower the SO2 load of untreated flue gas by 3.7–4.7% before an existing wet limestone flue gas desulfurization (WFGD) system. The combined desulfurization process with SO2 reduction saves the annual energy consumption by 4.0–5.0%, the environmental impacts by 3.0–5.0%, and the annual operating cost by 3.1–4.5%, compared to the existing WFGD process. The uncertainty analysis evaluates the probability of scenarios, and the sensitivity analysis recommends the parameter variations to further improve the combined desulfurization process and possible applications of the post SO2 capture fly ash. The approach can also be applied in other coal fired power plants for more environmentally-benign WFGD operation and cleaner electricity production.

Actual mercury speciation and mercury discharges from coal-fired power plants in Inner Mongolia, Northern China

Mercury species distribution in the flue gas from three typical coal-fired power plants have been investigated using a C-5000 sampler for Ontario Hydro Method (OHM) to evaluate specified Hg emission inventories in the largest coal-production province of China, Inner Mongolia. The feed coal, bottom ash, fly ash and gypsum have also been sampled in the field. Mercury emissions are in the dominant form of Hg0 (>85%). The Hg2+ (<15%) and the Hgp (<2%) emissions are minor fractions of Hgt in the flue gas. The wet limestone flue gas desulfurization device (WLFGD) showed much higher removal efficiency for Hg2+ (87–95%) than for the other two Hg species. The input Hg/output Hg varied from 92% to 115% for the tested 3 coal-fried utility boilers during sampling time. We obtained comprehensive Hg discharge factors, including direct atmospheric Hg emission factors and indirect discharge factors of Hg associated with bottom ash, fly ash and gypsum, to the environment, designating the fates of Hg in coal. We estimated the Hg discharges from coal-fired power plants in Inner Mongolia in 2008–2014 and expanded to China. The maximum of direct Hg emissions from coal-fired power plants of Inner Mongolia was 24Mg in 2012. The Chinese coal-fired power plants emitted directly 100Mg Hg to the atmosphere every year at the nationwide air control actions. The maximum discharges of Hg associated with fly ash and gypsum of Inner Mongolia was 81Mg and 11Mg in 2012, respectively. The national Hg discharges with fly ash and gypsum every year are respectively about 300Mg and 82Mg, majority of which may be reemitted to the atmosphere by the utilization industry. Therefore, mercury emissions from coal-fired power plants are not the largest single source of Hg to the atmosphere in normal operation of the current efficient denitrification, particles control and desulfurization devices. Instead, the utilization industry of fly ash and gypsum would be emitting more Hg to the atmosphere.

Comparative life cycle environmental assessment of flue gas desulphurization technologies in China

This study is a comparative life cycle assessment (LCA) for two widely used flue gas desulphurization (FGD) technologies in China, namely, the circulating fluidized bed flue gas desulfurization (CFB-FGD) and the wet limestone flue gas desulfurization (WFGD). The present study identifies and quantifies the energy consumptions, resource depletions and environmental impacts of CFB-FGD and WFGD technologies throughout the whole life cycle, including production of desulfurizer, production of FGD system manufacturing materials, transport of FGD system manufacturing materials, manufacturing of FGD system, transport of FGD system and FGD system operation. Life cycle assessment (LCA) methodology provides a quantitative basis for assessing potential improvements in the environmental performance of FGD technologies. The functional unit of the study is 7000 h, the annual operating time of FGD systems. The assessment criteria include five energy consumption categories (electricity, coal, diesel, fuel oil and crude oil), three resource consumption categories (coal, crude oil and water), and six environmental impact categories (global warming, acidification, nutrient enrichment, photochemical ozone formation, bulk waste and slag & ashes). Finally, the total energy consumptions, the resource depletion indexes (RDIs) and the environmental impact loadings (EILs) of FGD technologies were produced for the overall assessment. Overall, the WFGD technology is deemed environmentally benign and energy saving in a relative sense based on the LCA in this study. Concerning environmental impacts, bulk waste is the most critical for both types of FGD technologies. Further observations also help identify some sensitive parameters, including mixed solid waste and carbon monoxide (CO) across the CFB-FGD technology, and mixed solid waste and sulfur dioxide (SO

A simple realistic modeling of full-scale wet limestone FGD units

A model of wet limestone flue gas desulfurization (WLFGD) system has been developed based on the use of the number of transfer units (NTU) and height of transfer unit (HTU) focused to full-scale coal-fired power plants. The model describes the process occurring in the absorber as the most important step in the WLFGD system by establishing what increase in SO 2 removal can be achieved as the higher levels of nozzles come progressively into service when initially the first spray header is operating alone, and keeping the other operating conditions under control. This relative improvement of the SO 2 removal can be obtained from only one datum: the number of mass transfer units when only the first spray level is on service ( N 1). The model computes the gains in both the volumetric overall mass transfer coefficient and the number of transfer units. The model can be used to evaluate data or changes in an existing full-scale system as well as to evaluate proposed system designs and even as a pre-design tool in relation to the number of spray levels on service under controlled operating conditions. The model is tested on the basis of some data taken from a power plant and from the literature. The model results agree well with the real and published data.

Using Neural Networks to Address Nonlinear pH Control in Wet Limestone Flue Gas Desulfurization Plants

In previous articles,(1-3) we evaluated the performance of different linear control strategies (decentralized feedback control and multivariable predictive control) in wet limestone flue gas desulfurization (WLFGD) plants based on a pilot plant study. Although these control strategies show good performance, they may not be suitable if the oxidation tank pH is significantly nonlinear, which depends on many plant operating factors. Control of oxidation tank pH is important since it is related to the quality of gypsum, a byproduct of the WLFGD process. In this work, we propose and assess a combined control strategy for dealing with nonlinear pH control in WLFGD plants, based on a decentralized strategy composed of a neural predictive controller and a feedback controller, which control the oxidation tank pH and SO2 concentration of the desulfurized gas, respectively.

Model Predictive Control of a Wet Limestone Flue Gas Desulfurization Pilot Plant

A model predictive control (MPC) strategy based on a dynamic matrix (DMC) is designed and applied to a wet limestone flue gas desulfurization (WLFGD) pilot plant to evaluate what enhancement in control performance can be achieved with respect to a conventional decentralized feedback control strategy. The results reveal that MPC can significantly improve both reference tracking and disturbance rejection. For disturbance rejection, the main control objective in WLFGD plants, selection of tuning parameters and sample time, is of paramount importance due to the fast effect of the main disturbance (inlet SO2 load to the absorber) on the most important controlled variable (outlet flue gas SO2 concentration). The proposed MPC strategy can be easily applied to full-scale WLFGD plants.

Controllability Analysis and Decentralized Control of a Wet Limestone Flue Gas Desulfurization Plant

Presently, decentralized feedback control is the only control strategy used in wet limestone flue gas desulfurization (WLFGD) plants. Proper tuning of this control strategy is becoming an important issue in WLFGD plants because more stringent SO2 regulations have come into force recently.(3) Controllability analysis is a highly valuable tool for proper design of control systems, but it has not been applied to WLFGD plants so far. In this paper a decentralized control strategy is designed and applied to a WLFGD pilot plant taking into account the conclusions of a controllability analysis. The results reveal that good SO2 control in WLFGD plants can be achieved mainly because the main disturbance of the process is well-aligned with the plant and interactions between control loops are beneficial to SO2 control.

Dynamic Analysis and Identification of a Wet Limestone Flue Gas Desulfurization Pilot Plant

In order to meet more stringent SO2 regulations that have come into force recently, the operating conditions of wet limestone flue gas desulfurization (WLFGD) plants must be changed from design conditions to others where a higher SO2 removal is obtained; this results in higher operating costs. Furthermore, because of process disturbances, the SO2 removal target is usually set somewhat higher than necessary to provide a safety margin for meeting SO2 emission limits. Thus, the larger the safety margin, the higher the operating costs. How much larger the safety margin needs to be depends on the proper tuning of the control strategy and dynamic properties of the WLFGD plant to be controlled. Both aspects can only be studied if a dynamic model of the plant is available. However, dynamic modeling of WLFGD plants for control purposes has not been addressed in the literature so far. To deal with with this issue, in this paper we propose an identification methodology for control purposes applicable to full-scale plants. The identification methodology is put into practice in a WLFGD pilot plant, and the empirical dynamic model obtained is suitable for predicting the dynamic behavior of the pilot plant in a wide range of operating conditions. The dynamic behavior of the pilot plant is further analyzed from the perspective of the physical−chemical phenomena of the WLFGD process.

Technology Optimization of Wet Flue Gas Desulfurization Process

AbstractOptimization of the wet limestone (calcite) flue gas desulfurization (FGD) process is described, with the focus on the layout of subprocesses and equipment. The aim of optimization was to check the effectiveness and reliability of alternative layouts, involving intermediate product (gypsum) storage in the absorber make‐up tank, instant production of limestone slurry from powder in a wet screw conveyer and automatic gypsum dewatering line operation. Three alternatives, standard layout, optimized layout and optimized layout without reheating of treated gas, were designed and tested in a small industrial FGD plant. The comparison included evaluation of desulfurization efficiency, economical feasibility (considering a 125 MW power plant) and reliability of operation. The efficiency of the system was not affected by the layouts. Investment costs were reduced by 5 and 15 % for alternatives 1 and 2, respectively, compared to standard offers on the market. With respect to operating costs, variable operating and maintenance costs could be decreased by 0.5 up to 2.5 %, respectively. As shown by risk and reliability analysis, the availability of the system in both alternative layouts remained unchanged or was better with regard to the standard layout.

Optimum Values of Process Parameters of the “Wet Limestone Flue Gas Desulfurization System”

In this article the method of cost optimization of the “Wet Limestone Flue Gas Desulfurization System” is presented. The optimization calculations include process and cost models. The process model describes the most important stage of SO2 removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO2–3, dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO2 removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue gas desulfurization plants.

Optimum Values of Process Parameters of the “Wet Limestone Flue Gas Desulfurization System”

In this article the method of cost optimization of the “Wet Limestone Flue Gas Desulfurization System” is presented. The optimization calculations include process and cost models. The process model describes the most important stage of SO2 removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO2–3, dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO2 removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue gas desulfurization plants.

Flue Gas Desulfurization: The State of the Art

ABSTRACT Coal-fired electricity-generating plants may use SO2 scrubbers to meet the requirements of Phase II of the Acid Rain SO2 Reduction Program. Additionally, the use of scrubbers can result in reduction of Hg and other emissions from combustion sources. It is timely, therefore, to examine the current status of SO2 scrubbing technologies. This paper presents a comprehensive review of the state of the art in flue gas desulfurization (FGD) technologies for coal-fired boilers. Data on worldwide FGD applications reveal that wet FGD technologies, and specifically wet limestone FGD, have been predominantly selected over other FGD technologies. However, lime spray drying (LSD) is being used at the majority of the plants employing dry FGD technologies. Additional review of the U.S. FGD technology applications that began operation in 1991 through 1995 reveals that FGD processes of choice recently in the United States have been wet limestone FGD, magnesium-enhanced lime (MEL), and LSD. Further, of the wet limestone processes, limestone forced oxidation (LSFO) has been used most often in recent applications. The SO2 removal performance of scrubbers has been reviewed. Data reflect that most wet limestone and LSD installations appear to be capable of ~90% SO2 removal. Advanced, state-of-the-art wet scrubbers can provide SO2 removal in excess of 95%. Costs associated with state-of-the-art applications of LSFO, MEL, and LSD technologies have been analyzed with appropriate cost models. Analyses indicate that the capital cost of an LSD system is lower than those of same capacity LSFO and MEL systems, reflective of the relatively less complex hardware used in LSD. Analyses also reflect that, based on total annualized cost and SO2 removal requirements: (1) plants up to ~250 MWe in size and firing low- to medium-sulfur coals (i.e., coals with a sulfur content of 2% or lower) may use LSD; and (2) plants larger than 250 MWe and firing medium- to high-sulfur coals (i.e., coals with a sulfur content of 2% or higher) may use either LSFO or MEL.

Model of the Wet Limestone Flue Gas Desulfurization Process for Cost Optimization

A detailed process model of the wet limestone flue gas desulfurization system has been presented. This model can be used to calculate indispensable parameters for estimating costs and next to minimize capital and operating costs. The process model describes most important stage of SO2 removal running in an absorber and a holding tank. It includes absorption of sulfur dioxide, oxidation of SO3-, dissolution of limestone, and crystallization of gypsum. An assumption of thermodynamic equilibrium in the solution has been used. SO2 removal and limestone dissolution calculation has been based on stagnant-film theory. The model has been used for predicting the SO2 removal efficiency in the spray scrubber for process parameters, e.g., L/G, droplet diameter, stoichiometric ratio Ca/S, height of the absorption section, gas velocity, concentration in liquid-phase ions of Mg2+ and Cl-, and liquid pH. This model could be used to describe a multilevel spray system, too. The results of this model are in agreement with data from flue gas desulfurization installation at the Bełchatów Power Plant (Poland).