A. Talanker

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.

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

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

Increasing Operational Stability in Low NOX GT Combustor Using Fuel Rich Concentric Pilot Combustor

Abstract Lean combustion is a method in which combustion takes place under low equivalence ratio and relatively low combustion temperatures. As such, it has the potential to lower the effect of the relatively high activation energy nitrogen-oxygen reactions which are responsible for substantial NOX formation during combustion processes. However, lowering temperature reduces the reaction rate and deteriorates combustion stability. The objective of the present study is to reduce the lower equivalence ratio limit of the stable combustion operational boundary in lean Gas Turbine (GT) combustors while still maintaining combustion stability. A lean premixed gaseous combustor was equipped with a surrounding concentric pilot flame operating under rich conditions, thus generating a hot stream of combustion products with significant amount of reactive radicals. The main combustors fuel-air composition was varied from stoichiometric to lean mixtures. The pilots mixture composition was also varied by changing the air flow rate, within a limited rich mixtures range. The pilot fuel flow rate was always lower than five percent of the total fuel supply at the specific stage of the experiments.

Increasing Operational Stability in Low NOx GT Combustor by a Pilot Flame

The need for NOx reduction in gas turbine (GT) stimulates research for new combustion methods. Lean combustion is a method in which combustion takes place under low equivalence ratio and relatively low combustion temperatures. As such, it has the potential to lower the effect of the relatively high activation energy nitrogen-oxygen reactions which are responsible for substantial NOx formation during combustion processes. Moreover, lowering temperature reduces the reaction rate of the hydrocarbon-oxygen reactions and deteriorates combustion stability. The objective of the present study is to reduce the lower equivalence ratio limit of the stable combustion operational boundary in lean GT combustors. A lean premixed gaseous combustor was equipped with a surrounding concentric pilot flame operating under rich conditions, thus generating a significant amount of reactive radicals. The main combustor’s mixture composition was varied from stoichiometric to lean mixtures. The pilot’s mixture composition varied by changing the air flow rate, within a limited reach mixtures range. The pilot gas flow rate was always lower than five percent of the total gas supply at the specific stage of the experiments. The experiments and simulation showed that despite the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection by the pilot combustion has the potential to lower the limit of the global equivalence ratio (and temperatures) while maintaining stable combustion. Therefore the amount of generated NOx is expected to be significantly reduced as compared to a similar combustor of identical inlet and exit temperatures. In order to study the relevant mechanisms responsible for combustion stabilization, CFD and CHEMKIN simulations were performed to reveal the detailed flow characteristics and their spatial distribution within the combustor. Based on the CFD results, the CHEMKIN model was developed. The CHEMKIN simulations for atmospheric pressure showed satisfactory agreement with experimental results. Further simulation confirmed the advantageous of the technique also at elevated pressures. It is therefore important to understand the relevant mechanisms responsible for combustion stabilization and their spatial distribution within the combustor. The present work discusses an experimental- CFD-CHEMKIN combined approach aimed at studying the influence of radicals generated in the pilot ring combustion on the processes taking place in the main combustor.Copyright

Sub-Bituminous Coals Fired in Boiler Designed for Bituminous Coals

In the last two decades there has been little capacity added to coal-based power plants. However, much of the existing plants had to comply with the Clean Air Act amendments. Using sub-bituminous coals has become an important solution for emissions compliance due to their unique constituents and combustion characteristics; these coals are often referred to as enviro coals. The considerable advantages of these coals, like Melawan, Adaro or PRB coals, is their low sulfur compared to typical bituminous coals, which makes its burning more economic as scrubbers or other SO2 reduction technologies are not required. Low nitrogen and ash content as well as their high volatile matter are other advantages of these coals. Hence, firing sub-bituminous coals alone or as blends with bituminous coals is deemed economically attractive. Power generation plants were originally designed to operate on a particular bituminous coal. In order to fire sub-bituminous coals or their blends some modifications are required in the firing modes. These modifications may affect boiler reliability and as result to reduction of the power plant availability and hence increasing operation and maintenance cost. In order to prevent such undesirable effects we initiated a study to understand the influence of using sub-bituminous coals on the capacity, limitations of furnace size, heat transfer surfaces, firing systems, pulverizers, fans and airheaters. The present paper discusses issues connected with each of these issues on the combustion system. We also present recommendations for reliable burning of various sub-bituminous coals and their blends in a 575 MW tangentially-fired boiler. For example, we found that firing Indonesian sub-bituminous coals (Adaro and Melawan) considerably reduced NOx (30% reduction) and SOx (reduced to 200 mg/dNm3 @6%O2 ) emissions without post combustion measures. We also tested various blends of sub-bituminous coals with bituminous coals and found positive and negative synergism in these blends with regard to NOx emissions. We used in the present study a series of experiments in a test facility and computational fluid dynamic codes.Copyright

Application of Neural Network Combined With CFD Modeling and Combustion Tuning in Large Coal-Fired Boilers

The objective of the present work was to develop an optimization method for the prediction of the behavior of coals or coal blends in utility boilers, in order to specify the performance and pollutant emissions during the firing. Two methods have been used to study the performance of single coals or coal blends in power station boilers (1) experimental tests, where the coal/blend was fired in either a power station or in a test rig, and (2) use of coal combustion computational fluid dynamic (CFD). Here we will discuss both methods. We present experimental results, for 575 MWe tangentially-fired Combustion Engineering boilers of Israel Electric Corporation and 50 kWth test rig of Ben-Gurion University, that show the control of NOx and carbon content in fly ash (LOI). In addition to the experimental measurements we also established a large data base using a CFD code for a large spectrum of operational conditions. Validation of CFD results was made by comparison with both test rig and full-scale boilers measurements. Only after ensuring that good fit was obtained between experimental measurements and CFD results, was CFD used to establish the data base for coals/blends at a large spectrum of operational conditions. In some cases CFD was run for coals/blends never burned in the boiler, but burned in the test rig. The data obtained, experimental, showed that with tuning and modified nozzles NOx was considerably reduced: from 1200 to 570 mg/dNm3 @ 6% O2 for South African coal at full load. At partial loads NOx emission dropped from 1400 to about 800 mg/dNm3 @ 6% O2 . High volatile coals, such as Colombian and Indonesian, firing led to additional NOx reduction to around 400 mg/dNm3 @ 6% O2 at full load. A very large data base was obtained in this effort and brought us to the idea of extending it by using a neural network algorithm [1]. We used these data as a base for the development of a code based on neural network and a mathematical optimization algorithm. The code was primarily intended for use by the plant personnel for better tuning coal-fired boilers to reduce NOx and minimize heat rate. The neural network develops non-linear mapping functions between the outputs of NOx , heat rate, LOI, etc. and the controllable boiler input parameters. The mapping functions are then analyzed by the mathematical optimization algorithm and optimal boiler operating condition are identified. Further, based on networks and a mathematical optimization algorithm we found a proper Adaro and KPC (Indonesian coals) blend and operation condition that led to NOx emission reduction less than 400 mg/dnm3 in a 575 MWe tangentially firing unit with a conventional firing system. This result was verified in experimentally in the boiler. The results presented in this work clearly show that the developed method for reduction emission and performance optimization is available and capable to achieve operational or environmental goals.© 2006 ASME