Boris Chudnovsky

Fouling Formation in 575 MV Tangential-Fired Pulverized-Coal Boiler

Due to the liberalization of the energy markets and the globalization of coal procurement, fuel management became of substantial importance to power plant operators, which are faced with new challenges when operating with coal types different from the originally designed ones for the specific boiler. Environmental regulations, combustion behavior, possible malfunctions and low operation, and maintenance cost became of essential importance. Fouling is one of the major challenges when new coals are being used. For that purpose we initiated a comprehensive study of fouling on the water-wall tubes in a 575 MW tangential-fired pulverized-coal utility boiler. We developed a methodology to evaluate fouling propensity of coals and specifically tested two bituminous South African coals: Billiton-Prime and Anglo-Kromdraai. The methodology is based on the adherence of ash particles on the water walls. Adherence of the ash particle depends on the particle properties, temperature, and velocity vector at the boundary layer of the water walls. In turn, the flow and temperature fields were determined by computational fluid dynamics (CFD) simulations. For CFD simulations we also needed the combustion kinetic parameters, emissivity, and thermal resistance, and they were all determined experimentally by a 50 kW test facility. Using this methodology we mapped off the locations where fouling is mostly to occur. It was found that our results fitted with the experience from the data obtained,for these two coals in the Israel Electric Corporation utility boilers. The methodology developed was shown to be able to provide the fouling propensity of a certain coal, and yielded good prediction of the fouling behavior in utility boilers. Therefore, the methodology can assist in the optimization of the soot-blowing regime (location and frequency).

Testing and Prediction of Performance and Emissions From Bituminous (Drummond Colombia) and Sub-Bituminous (Adaro Indonesia) Coals and Their Blends

We predict the combustion behavior and pollutant emissions of blends of a Colombian bituminous coal, Drummond, and an Indonesian sub-bituminous coal, Adaro, in pulverized-coal utility boilers. This work is based on full-scale numerical simulations with GLACIER, a powerful computational-fluid-dynamic (CFD) code that uses the two-mixture fraction approach which models two separate coal streams in the combustion chamber. By burning the coals and their blends in a pilot-scale test furnace, previously unknown information on the coal combustion, such as devolatilization and char oxidation kinetic parameters, was determined and the CFD model validated for the test furnace. The same set of parameters was used for the CFD model configured for an opposed-wall and a tangential fired utility boiler. Our results show good fits between numerical results and experimental data for gas temperature, CO2 , O2 , and NOx , both in the test furnace and in the utility boilers, for single coals and their blends. We believe that the tool we developed can help utility companies make rational decisions on the use of new coals or coal blends so as to lower pollutant emissions while maintaining the same combustion efficiency.Copyright

Extension of the Combustion Stability Range in Dry Low NOx Lean Premixed Gas Turbine Combustor Using a Fuel Rich Annular Pilot Burner

The present work is concerned with improving combustion stability in lean premixed (LP) gas turbine combustors by injecting free radicals into the combustion zone. The work is a joint experimental and numerical effort aimed at investigating the feasibility of incorporating a circumferential pilot combustor, which operates under rich conditions and directs its radicals enriched exhaust gases into the main combustion zone as the means for stabilization. The investigation includes the development of a chemical reactors network (CRN) model that is based on perfectly stirred reactors modules and on preliminary CFD analysis as well as on testing the method on an experimental model under laboratory conditions. The study is based on the hypothesis that under lean combustion conditions, combustion instability is linked to local extinctions of the flame and consequently, there is a direct correlation between the limiting conditions affecting combustion instability and the lean blowout (LBO) limit of the flame. The experimental results demonstrated the potential reduction of the combustion chamber’s LBO limit while maintaining overall NOx emission concentration values within the typical range of low NOx burners and its delicate dependence on the equivalence ratio of the ring pilot flame. A similar result was revealed through the developed CHEMKIN-PRO CRN model that was applied to find the LBO limits of the combined pilot burner and main combustor system, while monitoring the associated emissions. Hence, both the CRN model, and the experimental results, indicate that the radicals enriched ring jet is effective at stabilizing the LP flame, while keeping the NOx emission level within the characteristic range of low NOx combustors. [DOI: 10.1115/1.4026186]

Prediction of Performance From PRB Coal Fired in Utility Boilers With Various Furnace and Firing System Arrangements

The present regulatory requirements enforce the modification of the firing modes of existing coal-fired utility boilers and the use of coals different from those originally designed for these boilers. The reduction in SO 2 and NO x emissions was the primary motivation for these changes. Powder river basin (PRB) coals, classified as subbituminous ranked coals, can lower NO x and SO x emissions from power plants due to their high volatile content and low sulfur content, respectively. On the other hand, PRB coals have also high moisture content, low heating value, and low fusion temperature. Therefore when a power plant switches from the designed coal to a PRB coal, operational challenges were encountered. A major problem that can occur when using these coals is the severe slagging and excess fouling on the heat exchanger surfaces. Not only is there an insulating effect from deposit, but there is also a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction in heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGTs), especially with opposite wall burners and with a single backpass. Higher FEGTs usually result in higher stack gas temperature, increasing the reheater spray flow and therefore decreasing the boiler efficiency with a higher heat rate of the unit. A successful modification of an existing unit for firing of PRB coals requires the evaluation of the following parameters: (1) capacities or limitations of the furnace size, (2) the type and arrangement of the firing system, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to make this evaluation for three PRB coals to be potentially fired in a 575 MW tangential-fired boiler.

Fouling Formation in 575 MW Tangential-Fired Pulverized-Coal Boiler

Over the past years we have gained experience in employing varying types of coal fired in the same boiler. Consequently, we developed evaluation criteria regarding the operation of these coals. In the present study we evaluated fouling propensity of two bituminous South African coals Billiton-Prime and Anglo-Kromdraai (AKD). From an experimental work carried out in a 50 kW test facility where we obtained emissivity and thermal resistance of the two coal ashes. We also developed a new method for evaluating fouling in full-scale utility boilers that is based on adhering of an ash particle depending on its temperature and velocity vector at the boundary layer of the water-walls. The flow and temperature fields were determined by computational fluid dynamic (CFD) simulations. We used the emissivity and thermal resistance determined from the test furnace in the CFD calculations to evaluate fouling in the combustion chamber of a 575 MW tangential-fired utility boiler when the two bituminous coals were fired. Mapping of the fouling locations that are mostly to occur were determined and this behavior fit the experience from the power station and data obtained for these two coals.Copyright

Advanced Supervision and Diagnostic System: The Tool for Improvement Reliability and Efficiency of Utility Boilers Burned Different Coals Type

Due to the past years experience, we have seen an influence on the coal purchase policies. The main reason for using coal other tan the original design coal, apart from economic considerations, is to reduce SO2 emission by using a low sulfur content coal as well as decreasing coal index (fixed carbon to volatile matter ratio) in order to reduce NOx emission etc. Hence, the power generation plant was originally designed to operate on a particular coal, and due to high restriction in environmental requirements, emissions regulations, have led to modifications of the coal type and firing mode, while the boilers must burn a different coal type and its blends. It may lead to boiler reliability degradation and as a result reducing the power plant availability and increasing operating and maintenance cost. In order to prevent a different faults in boiler operation and as a result a reliability reduction, special monitoring and diagnostic techniques is required for engineering analysis and utility production management. In this sense an on-line supervision system have developed and implemented for 575, 550, 360 Mw coal fired units. The aim of the system is to achieve more effective and reliable operation with emission reduction. It is a valuable aid for: (1) operation staff and process engineers to survey the equipment and overall plant performance, (2) electricity production management in order to implement maintenance and operation strategies. The developed system provides an on-line information regards furnace performance, including fireball location, increase in furnace exit temperature indicates that it’s necessity to activate furnace sootblowers to control steam temperatures and prevent excessive accumulations of slag and heat surfaces tube overheating. The developed system indicates the superheater and reheater surfaces midwall metal temperature which provides timely information to assist engineering personnel to optimize mode of operation, in order to activate the optimal number of sootblowing of the corresponded surfaces, etc. The system includes also an online analysis of the erosion impact on the boiler backpass heat surfaces. By the using the developed system we found a correlation between furnace fouling and NOx emission due to the effect of flame temperature on the rate of NOx formation. As a result an optimal mode of furnace operation and sootblowing practice is used to reduce the high cost of controlling NOx emission. The developed system allows to fond the proper coal blends which provide a high boiler efficiency and reliability while minimizing NOx emissions.© 2007 ASME

Prediction of Performance and Pollutant Emission From Bituminous and Sub-Bituminous Coals in Utility Boilers

Computational Fluid Dynamic (CFD) models give good predictions of coal combustion in utility boilers if the coal combustion kinetic parameters are known. We developed a three-step methodology to provide reliable prediction of the behavior of a coal in a utility boiler: (1) Obtaining the combustion kinetic model parameters from a series of experiments in a test facility, CFD codes and optimization algorithm. (2) Validation of the combustion kinetic parameters by comparison of different experimental data with simulation results obtained by the set of combustion kinetic parameters. (3) The extracted kinetic parameters are then used for simulations of full-scale boilers using the same CFD code. Three to four bituminous and sub-bituminous coals with known behavior in Israel Electric Corporation (IEC) 550MW opposite-wall (3 coals) and 575MW tangential-fired (4 coals) boilers were used to show the capability of the method. An unfamiliar bituminous coal was then examined prior of its firing in the utility boilers and prediction of its combustion behavior in the two boilers was carried out. This methodology was used to examine a Venezuelan coal that was found to yield high LOI.Copyright

LOWERING EQUIVALENCE RATIO LIMIT FOR STABLE COMBUSTION IN GAS TURBINES BY INJECTION OF FREE RADICALS

Lean premixed combustion is one of the widely used methods for NOx reduction in gas turbines (GT). When this method is used combustion takes place under low Equivalence Ratio (ER) and at relatively low combustion temperature. While reducing temperature decreases NOx formation, lowering temperature reduces the reaction rate of the hydrocarbon–oxygen reactions and deteriorates combustion stability. The objective of the present work was to study the possibility to decrease the lower limit of the stable combustion regime by the injection of free radicals into the combustion zone. A lean premixed gaseous combustor was designed to include a circumferential concentric pilot flame. The pilot combustor operates under rich fuel to air ratio, therefore it generates a significant amount of reactive radicals. The experiments as well as CFD and CHEMKIN simulations showed that despite of the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection from the pilot combustor has the potential to lower the limit of the global ER (and temperatures) while maintaining stable combustion. Spectrometric measurements along the combustor showed that the fuel-rich pilot flame generates free radicals that augment combustion stability. In order to study the relevant mechanisms responsible for combustion stabilization, CHEMKIN simulations were performed. The developed chemical network model took into account some of the basic parameters of the combustion process: ER, residence time, and the distribution of the reactances along the combustor. The CHEMKIN simulations showed satisfactory agreement with experimental results.Copyright

Prediction of Fouling and Slagging in Pulverized-Coal Fired Furnaces

The objective of this paper is to develop a method for predicting the fouling dynamics and thermal resistance of a certain coal and its fly ash in a coal-fired utility boiler as well as determine the emissivity of the fly ash in order to optimize soomblowing procedures for fouling clean-up. The methodology comprises combustion experiment in a 50kW test facility, characterization of the coal and its mineral matter and the consequent fly ash as well a series of computational fluid dynamic simulations. Ash deposition from Cerrejon D Colombian and Billiton-Prime South African coals were studied and their behavior predicted in 575MW tangentially-fired and 550MW opposite-wall boilers. Good comparison was obtained between predicted results and actual data from the boilers.Copyright

Evaluation of Biomass and Torrefied Coal Co-Firing in Large Utilities Boilers

Renewable energy targets and CO2 emissions markets drive the transition to a cleaner and renewable energy production system. In this manner, utilities are looking for cost effective options with a minimum impact on unit performance and reliability. Co-firing biomass, in comparison with other renewable sources, is the main contributor to meeting the world’s renewable energy target. It avoids the destruction of capital, by making coal-fired power plants cleaner without having to replace them. Biomass co-firing provides a relatively low cost means of increasing renewables capacity and an effective way of taking advantage of the high thermal efficiency of large coal fired boilers. The direct displacement of coal when co-firing plus the higher conversion efficiencies generally achieved also contribute to achieving higher CO2 reduction benefits from each co-fired tone of biomass. However, coal–fired power plants are not designed to co-fire large amounts of biomass. This means that not more than 5–10% of biomass can be co-fired. In order to increase this amount, utilities have to make significant investments in dedicated biomass handling and processing equipment. Even when these investments are made, the co-firing percentage is often limited to 20% thermal fraction, because the chemical and physical properties of bio-fuels. Another possibility, to increase biomass fraction in co-firing is torrefied fuel burning. Co-firing torrefied biomass could increase considerably co-firing percentages, while saving investment and transport cost compared to biomass co-firing. However, it should be concerned regarding the ability of generators involved in coal and biomass co-firing that this alternative may impact on boiler reliability due to specific biomass properties and it this issue should be carefully evaluated during design stage. In order to prevent such an undesirable effect we initiated a study to understand the influence of using co-firing on the capacity, limitations of furnace size, heat transfer surfaces, firing systems, pulverizers, fans, airheaters and equipment for post combustion emission treatment.This paper discusses the technical and commercial application of coal and biomass/ torrefied coal co-firing in large utility boilers. In the present study we used a series of simulation using computer codes; the latter are CFD codes suitable for simulation of the performance and emissions of co-fired utility boilers and an expert system that aided in issues like boiler and furnace performance, pulverizing capabilities, post combustion treatment equipment performance, sootblowing optimization, boiler Fans operation and performance.Copyright