Coal-fired Combustion Research Articles

ANN - SQP Approach For NOx Emission Reduction In Coal Fired Boilers

Abstract In this research paper, predictive modelling of NOx emission of a 210 MW capacity pulverized coal-fired boiler and combustion parameter optimization to reduce NOx emission in flue gas is proposed. The effects of oxygen concentration in flue gas, coal properties, coal flow, boiler load, air distribution scheme, flue gas outlet temperature and nozzle tilt were studied. The data collected from parametric field experiments were used to build a feed-forward back-propagation artificial neural net (ANN). The coal combustion parameters were used as inputs and NOx emission as outputs of the model. The ANN model was developed for full load condition and its predicted values were verified with the actual values. The algebraic equation containing weights and biases of the trained net was used as fitness function in sequential quadratic programming (SQP) to find the optimum level of input operating conditions for low NOx emission. The results proved that the proposed approach could be used for generating feasible operating conditions.

Stability Experimental Study on High Temperature and Oxygen-Containing Flue Gas

During lignite pyrolysis process, there will produce large amounts of gas which is low calorific value, combustion difficult. This paper creatively put forward a program: high temperature and oxygen-containing flue gas of coal-fired mix combustion with low calorific value gas. Compared to mixing high calorific value fuel, the distinguishing feature of this program is ultra-low cost, and technically feasible. In this scheme, the stability of high-temperature and oxygen-containing flue gas is key technology. Through coal-fired experiments, we obtained generation rule of high-temperature and oxygen-containing flue gas, and summed up a diminishing marginal effect.

The importance of the location of sodium chlorite application in a multipollutant flue gas cleaning system

In this study, removing sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury (Hg) from simulated flue gas was investigated in two laboratory-sized bubbling reactors that simulated an oxidizing reactor (where the NO and Hg0 oxidation reactions are expected to occur) and a wet limestone scrubber, respectively. A sodium chlorite solution was used as the oxidizing agent. The sodium chlorite solution was an effective additive that enhanced the NOx, Hg, and SO2 capture from the flue gas. Furthermore, it was discovered that the location of the sodium chlorite application (before, in, or after the wet scrubber) greatly influences which pollutants are removed and the amount removed. This effect is related to the chemical conditions (pH, absence/presence of particular gases) that are present at different positions throughout the flue gas cleaning system profile. The research results indicated that there is a potential to achieve nearly zero SO2, NOx, and Hg emissions (complete SO2, NO, and Hg removals and ∼90% of NOx absorption from initial values of 1500 ppmv of SO2, 200 ppmv of NOx, and 206 μg/m3 of Hg0) from the flue gas when sodium chlorite was applied before the wet limestone scrubber. However, applying the oxidizer after the wet limestone scrubber was the most effective configuration for Hg and NOx control for extremely low chlorite concentrations (below 0.002 M) and therefore appears to be the best configuration for Hg control or as an additional step in NOx recleaning (after other NOx control facilities). The multipollutant scrubber, into which the chlorite was injected simultaneously with the calcium carbonate slurry, appeared to be the least expensive solution (when consider only capital cost), but exhibited the lowest NOx absorption at ∼50%. The bench-scale test results presented can be used to develop performance predictions for a full- or pilot-scale multipollutant flue gas cleaning system equipped with wet flue gas desulfurization scrubber. Implications The control of NOx, Hg, and SO2 emissions from coal-fired combustors is an important, worldwide concern affecting environmental pollution. In both the European Union and the United States, NOx and SO2 emissions are subject to newly tightened emission limits. This paper investigated the NOx, Hg, and SO2 removals in a flue gas cleaning system equipped with a classical wet limestone scrubber, enhanced by a solution of sodium chlorite. This paper demonstrated that the location at which the sodium chlorite is applied in a flue gas cleaning system affects the control of these pollutants and discusses how possible solutions could be practically implemented.

Predictive modelling and optimization of nitrogen oxides emission in coal power plant using Artificial Neural Network and Simulated Annealing

In this research paper, predictive modelling of NOx emission of a 210 MW capacity pulverized coal-fired boiler and combustion parameter optimization to reduce NOx emission in flue gas is proposed. The effects of oxygen concentration in flue gas, coal properties, coal flow, boiler load, air distribution scheme, flue gas outlet temperature and nozzle tilt are studied. The data collected from parametric field experiments are used to build a feed-forward back-propagation artificial neural net (ANN). The coal combustion parameters are used as inputs and NOx emission as outputs of the model. The ANN model is developed for full load condition and its predicted values are verified with the actual values. The algebraic equation containing weights and biases of the trained net is used as fitness function in simulated annealing (SA) to find the optimum level of input operating conditions for low NOx emission. The result proves that the proposed approach could be used for generating feasible operating conditions. DOI: http://dx.doi.org/10.11591/ij-ai.v1i1.286 Full Text: PDF

Effects of participating media on the time-resolved infrared measurements of wall temperature in a coal-fired combustor

The infrared temperature fluctuations on a plate in a coal-fired combustor were measured and the effects of participating media on infrared measurement were revealed. The experimental results were analyzed in both the time and frequency domains. It is found the fluctuations have both random and periodic characteristics. The periodic frequency is identified to be caused by vortex shedding of the gas-particle two-phase flow around a cylinder. This conclusion is supported by a numerical analysis.

Mercury policy and regulations for coal-fired power plants

Mercury is a high-priority regulatory concern because of its persistence and bioaccumulation in the environment and evidence of its having serious adverse effects on the neurological development of children. Mercury is released into the atmosphere from both natural and anthropogenic sources. Coal-fired utilities are considered to be one of the largest anthropogenic mercury emission sources. The period since the late 1990s has been marked by increasing concern over mercury emissions from combustion systems to the extent that a number of national governments have either already implemented or are in the process of implementing, legislation aimed at enforcing tighter control over mercury emissions and a reduction in mercury consumption. This review examines the most important national and international policies and agreements for controlling mercury emissions from coal-fired combustion systems. To provide a global perspective, this study lists the countries with the largest estimated mercury emissions and regulatory efforts to reduce them.

Conceptual feasibility study of retrofitting coal-fired power plant with oxy-fuel combustion

Capturing and sequestering carbon dioxide (CO2) from coal-fired power plant has been proposed as one of the key solutions for stabilizing greenhouse gas concentration in the atmosphere. There are two main procedures for CO2 capturing in existing coal-fired combustion plants: removal of nitrogen from flue gases and removal of nitrogen from the air before combustion in order for a gas stream to be ready for geo-sequestration. In oxy-fuel combustion, fuel is burned with pure oxygen (O2) instead of air. This technology recycles flue gases back into the furnace to control temperature and make up the volume of the missing nitrogen (N2) to ensure that there is sufficient gas to maintain the temperature and heat flux profiles in the boiler. In this study, feasibility of oxy–coal fired for retrofitting existing normal air-operated coal-fired power plant has been studied. Also, the optimum fraction of recirculated flue gas for maximizing generated power is represented. Our results show that the oxy-fuel combustion is suitable for retrofitting the existing coal-fired power plants as well as for new plants without the replacement of main components.

Mercury control technologies for coal combustion and gasification systems

Development and testing of mercury control technologies have largely focused on coal-fired combustion systems, with less emphasis on advanced power systems. Mercury control is influenced by coal properties and chemistry, plant configuration, pollution control devices, flue gas conditions, and plant operations, which differ between combustion and gasification systems. Sorbents such as treated activated carbons have shown promising results in low-temperature environments; however, elevated temperature and reducing environments of many advanced systems remain challenging, requiring research and development to obtain acceptable mercury control levels. Concurrent pollutant/multipollutant control strategies that include CO 2 control are critically needed for both conventional and advanced power systems.

A simple measurement method of temperature and emissivity of coal-fired flames from visible radiation image and its application in a CFB boiler furnace

The two-color method is widely used in non-contact temperature measurement of combustion flames from radiation images, traditionally based on the spectroscopic characteristics of the image sensor, and/or the representative wavelengths for the red, green and blue filters of the image-forming devices, for example, Charge Coupled Device (CCD) cameras. In this paper, a new method to derive the temperature and emissivity images from a color CCD image is presented for coal-fired combustion processes, which is independent of the spectroscopic characteristics and the representative wavelengths of the CCD and the image processing system. In this method, it is only necessary to capture image information by the image processing system from a blackbody furnace at different temperatures which cover the possible temperature range of the combustion processes to be measured. In a 480 ton/h coal-fired circulating fluidized bed (CFB) boiler experiments are conducted. The temperature measured by this method is validated by that obtained by thermocouple, and it varies obviously with the load the boiler. The flame emissivity provided by this system is worthy for further study.

Study on Mercury Exposure of Primary School Children Near by Coal-Fired Power Plants in Korea

ISEE-713 Objective: Mercury is highly toxic and may be fatal if inhaled. It is harmful if absorbed through the skin or consumed with food. Mercury may cause harmful effects to the neural, digestive, respiratory, immune systems, and kidney on human health. This study was conducted to study the influence of existing coal-fired power plant, which is known to emit mercury artificially, on body mercury concentration levels, using biological samples of elementary school students who live in and around the coal-fired power plant area and control area. Materials and Methods: The mercury concentrations of children who live near the power plant were investigated to survey the influence of coal-fired combustion on the exposure. 3 main power plant areas and 4 control areas were selected for the investigation. Mercury analyzer using gold amalgam collection method with thermal decomposition was used. Total mercury analysis in the whole blood and urine samples and questionnaire information about the subjects’ dietary habit were analyzed as well. Results: The exposure levels of children in power plant area were 2.13 μg, 1.84 μg-creatinine in blood and urine respectively. Those of children in control area were 2.20 μg, 2.47 μg-creatinine. Different from general presumption, mercury level in control area was higher. Any obvious evidences that emissions from the power plants influenced on the mercury exposure level were not found out. Conclusions: On this study, several investigations related to health effects were conducted to evaluate the influence of mercury exposure but any special evidences were not found. Food-related factors such as preference for fish and frequency of fish intake were found to be the main sources of mercury exposure in domestic children.

Diode-laser-based sensor for ultraviolet absorption measurements of atomic mercury

A new sensor has been developed for measuring atomic mercury using absorption spectroscopy with 254-nm radiation generated from two sum-frequency-mixed diode lasers. Beams from a 375-nm external-cavity diode laser and a 784-nm distributed feedback diode laser are mixed in a beta-barium-borate crystal to generate approximately 4 nW of ultraviolet radiation. The development of the sensor is described along with extensive characterization experiments in a mercury vapor cell in the laboratory. An accuracy of ±6% in the absolute concentration of atomic mercury has been demonstrated by comparison with equilibrium vapor pressure calculations. The detection limit is approximately 0.1 parts per billion of atomic mercury in a meter path length for 300-K gas and a 10-s integration time. The insensitivity of the sensor to broadband attenuation is demonstrated. Measurements of collision-broadening coefficients for air, N2, Ar, and CO2 are reported, and implementation of wavelength-modulation spectroscopy with the sensor is demonstrated. Finally, results are presented from measurements with the sensor in situ in the exhaust stream of an actual coal-fired combustor.

A technique to control mercury from flue gas: The Thief Process

The Thief Process is a mercury removal process that may be applicable to a broad range of pulverized coal-fired combustion systems. This is one of several sorbent injection technologies under development by the U.S. Department of Energy for capturing mercury from coal-fired electric utility boilers. A unique feature of the Thief Process involves the production of a thermally activated sorbent in situ at the power plant. The sorbent is obtained by inserting a lance, or thief, into the combustor, in or near the flame, and extracting a mixture of partially combusted coal and gas. The partially combusted coal or sorbent has adsorptive properties suitable for the removal of vapor-phase mercury at flue gas temperatures that are typical downstream of a power plant preheater. One proposed scenario, similar to activated carbon injection (ACI), involves injecting the extracted sorbent into the downstream ductwork between the air preheater and the particulate collection device of the power plant. Initial laboratory-scale and pilot-scale testing, using an eastern bituminous coal, focused on the concept validation. Subsequent pilot-scale testing, using a Powder River Basin (PRB) coal, focused on the process development and optimization. The results of the experimental studies, as well as an independent experimental assessment, are detailed. In addition, the results of a preliminary economic analysis that documents the costs and the potential economic advantages of the Thief Process for mercury control are discussed.

Temperature Profiling of Pulverized Coal Flames Using Multicolor Pyrometric and Digital Imaging Techniques

This paper presents an imaging-based multicolor pyrometric system for the monitoring of temperature and its distribution in a coal-fired flame. A novel optical splitting/filtering device is designed and used to split the light of flame into three beams at three selected wavelengths as required in the multicolor principle. A high-resolution charge-coupled device camera is employed to collect the three beams of the light of flame. The three resulting images provide the basis for the determination of temperature and its distribution in the flame field. The system is evaluated on a 0.5-MW/sub th/ coal-fired combustion test facility under various combustion conditions. Results obtained demonstrate that the system is capable of measuring the temperature and its distribution concurrently in the flame field. Quantitative relationships between the measured results and the main combustion process data are also discussed.

Monitoring of Oscillatory Characteristics of Pulverized Coal Flames Through Image Processing and Spectral Analysis

This paper presents the monitoring of the oscillatory characteristics of pulverized coal flames using image processing and spectral analysis techniques. The instrumentation system employed in this investigation is an integral part of a multifunctional flame monitoring system, being capable of monitoring the oscillatory frequency of a flame on a two-dimensional and concurrent basis. A quantitative flicker frequency is defined as the power-density-weighted mean frequency over the spectral range to represent the oscillatory characteristics of a specific region of the flame. Digital filtering techniques incorporating direct gray-level thresholding and wavelet shrinkage algorithms are employed to reduce background noise from flame images and white noise from the resulting flame frequency signal. A series of tests was undertaken on an industrial-scale coal-fired combustion test facility (CTF) under a range of operating conditions. Relationships between the measured flame oscillatory frequency and the process data including emissions are identified. Results obtained demonstrate that the flame oscillatory frequency responds in predictable ways to the effects of operating conditions on the dynamic nature of the flame.

Mercury detection at boron doped diamond electrodes using a rotating disk technique

Quantification of mercury ions at the ppt level is reported using highly boron-doped diamond (BDD) film electrodes by differential pulse voltammetry (DPV). The DPV experiments were performed in nitrate, thiocyanate and chloride media. Investigation in chloride medium is important since practical samples usually contain chloride impurities. The formation of calomel in a chloride medium on the BDD surface is avoided by the co-deposition of purposely-added gold (3 ppm) during DPV detection. Excellent linear calibration plots have been obtained in all media for ppb ranges. Mercury in the 0.005–50 ppb range has been detected using a rotating disk electrode (RDE) technique in real samples (KCl impinger solutions) prepared from flue gas released by a pilot-scale coal-fired combustion facility. A portable instrument has also been used for the detection of mercury efficiently. These studies have demonstrated that BDD mounted in an RDE system together with gold co-deposition is able to detect mercury with sufficient sensitivity for practical analysis of environmental samples.

Emissions of Sulfur Trioxide from Coal-Fired Power Plants

Emissions of sulfur trioxide (SO3) are a key component of plume opacity and acid deposition. Consequently, these emissions need to be low enough to not cause opacity violations and acid deposition. Generally, a small fraction of sulfur (S) in coal is converted to SO3 in coal-fired combustion devices such as electric utility boilers. The emissions of SO3 from such a boiler depend on coal S content, combustion conditions, flue gas characteristics, and air pollution devices being used. It is well known that the catalyst used in the selective catalytic reduction (SCR) technology for nitrogen oxides control oxidizes a small fraction of sulfur dioxide in the flue gas to SO3. The extent of this oxidation depends on the catalyst formulation and SCR operating conditions. Gas-phase SO3 and sulfuric acid, on being quenched in plant equipment (e.g., air preheater and wet scrubber), result in fine acidic mist, which can cause increased plume opacity and undesirable emissions. Recently, such effects have been observed at plants firing high-S coal and equipped with SCR systems and wet scrubbers. This paper investigates the factors that affect acidic mist production in coal-fired electric utility boilers and discusses approaches for mitigating emission of this mist.

Behavior of selenium in the combustion of coal or coke spiked with Se

Selenium (Se) is one of the most volatile trace metals emitted from coal-fired combustors and will most probably be considered for regulation in the electric power industry due to its significant environmental impact. Novel technologies are needed to remove Se vapor. This requires first a clear understanding of the behavior of selenium in combustion. In this work, coal and coke samples spiked with Se were prepared and the effect of Se on combustion was evaluated in a thermobalance. Impregnation with Se had a negligible impact on the combustion of coke; this allowed a systematic experimental study of the vaporization, condensation, and association of Se through burning cokes spiked with Se. Capturing the aerosol in the combustion flue gas on a membrane filter, coupled with identification of the species present using X-ray photoelectron spectroscopy, was established as a useful approach for coal combustion. The high volatility of Se and its affinity for oxygen, as well as the effect of chlorine on its vaporization, supported our previous calculations. The dominant Se species were identified as atomic Se and SeO 2, which also confirmed the thermodynamic predictions.

Recent advances in mercury removal technology at the National Energy Technology Laboratory

The in-house research effort on mercury studies at the National Energy Technology Laboratory is an integrated, multi-task approach, including experimentation at both the laboratory-scale and the pilot-scale, as well as a numerical modeling effort to aid in interpretation of pilot-scale results. In the laboratory-scale work, novel sorbents and techniques for the removal of mercury from flue gas are being investigated using a small packed-bed reactor. The reactor system is used to screen novel sorbents for their capability to remove mercury from gas streams. The capacities of these novel sorbents are determined as a function of gas composition and temperature and are compared to results with commercially available activated carbons. In the pilot-scale work, an existing pilot unit has been characterized with respect to the distribution and fate of hazardous air pollutants in flue gas, with an emphasis on mercury. The pilot unit is a 500-lb/h (227-kg/h) pulverized coal-fired (PCFC) combustion system that includes a furnace, air preheater, spray dryer, ductwork and a pulse-jet fabric filter. The investigations with this unit have entailed evaluation of various activated carbons and novel sorbents, as well as comparisons of various sampling techniques for the determination of total and speciated forms of mercury. The impact of various parameters (temperature, sorbent-to-mercury ratio, baghouse pressure drop) on the removal of mercury has been investigated. Additionally, a slipstream allows for the investigation of sorbent injection in a fly ash-free flue gas over a wide range of operating conditions, including residence time. A summary of the results for mercury measurement and control from the 500-lb/h (227-kg/h) PCFC combustion system will be presented, as well as the evaluation of methods for measurement of in-duct removals using the slipstream. Finally, a discussion of the numerical modeling effort will be given.

Calcium magnesium acetate and urea advanced reburning for NO control with simultaneous SO 2 reduction

Calcium magnesium acetate (CMA) shows potential as a reductant for simultaneous NO x and SO x removal from coal-fired combustion plant. The performance of urea co-injection with CMA on NO reduction in an ‘advanced reburn’ (AR) configuration has been investigated with a view to optimization of the process in a pulverized coal-fired furnace operating at 80 kW. The impact on SO 2 reduction has also been investigated. Urea/CMA solution was sprayed into the reburn zone of the furnace using twin-fluid atomizers over a range of reductant/NO stoichiometric ratios (NSR). The influence on NO reductions of primary zone stoichiometry ( λ 1) was investigated for a range of CMA reburn feed rates (Rff) and reburn zone stoichiometry ( λ 2). In addition, the effect of temperature on the SNCR performance of urea was investigated. Optimum process conditions were categorized either by maximizing NO and SO 2 reductions (Modes A and B, respectively) or maximizing reductant utilization efficiencies (Modes C and D). NO control was best performed at λ 1=1.05, but SO 2 reductions were greatest at more fuel-lean primary zone conditions ( λ 1=1.15). Highest NO reductions of 85% under AR-rich conditions were achieved under Mode A, but were only slightly higher compared with reductions of 79% under Mode B, where SO 2 reductions were optimized at 85%. N-utilization was also at an acceptable level of 25% compared to the maximum utilization efficiency which was obtained at NSR=1.5 of 30% for the same conditions of stoichiometry operating in Mode C. Operation at this lower level of reburn (9.6%) could significantly reduce the consumption of CMA with some impact on NO reduction (73%). SO 2 removal performance would be compromised severely with reductions lowered from 75% at Mode A to 35% at Mode C. Optimizing Ca utilization (Mode D) resulted in poor NO and SO 2 reductions, at 61 and 22%, respectively, and can be discounted as a viable option. The technique offers flexibility of operation depending on the emission control requirements.

Control of mercury vapor emissions from combustion flue gas.

Mercury (Hg) emission from combustion flue gas is a significant environmental concern due to its toxicity and high volatility. A number of the research efforts have been carried out in the past decade exploiting mercury emission, monitoring and control from combustion flue gases. Most recently, increasing activities are focused on evaluating the behavior of mercury in coal combustion systems and developing novel Hg control technologies. This is partly due to the new regulatory requirement on mercury emissions from coal-fired combustors to be enacted under the U.S. Title III of the 1990 Clean Air Act Amendments. The aim of this review work is to better understand the state-of-the-art technologies of flue gas mercury control and identify the gaps of knowledge hence areas for further opportunities in research and development. This paper examines mercury behaviors in combustion systems through a comprehensive review of the available literature. About 70 published papers and reports were cited and studied. This paper summarizes the mechanisms of formation of mercury containing compounds during combustion, its speciation and reaction in flue gas, as well as subsequent mobilization in the environment. It also provides a review of the current techniques designed for real-time, continuous emission monitoring (CEM) for mercury. Most importantly, current flue gas mercury control technologies are reviewed while activated carbon adsorption, a technology that offers the greatest potential for the control of gas-phase mercury emissions, is highlighted. Although much progress has been achieved in the last decade, techniques developed for the monitoring and control of mercury from combustion flue gases are not yet mature and gaps in knowledge exist for further advancement. More R&D efforts are required for the effective control of Hg emissions and the main focuses are identified.