Roman Saveliev

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

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

Fouling of Heat Exchanger Tubes in Pulverized-Coal-Fired Combustion Chambers

Fouling is a major concern in coal-fired power plants caused by fly ash deposit on the heat exchanger tubes that decreases the overall heat transfer coefficient to water-steam mixture. Fouling has been characterized by weakly bound-loose form, which may be removed by various methods, such as soot-blowing, blast, and sand blowing. We have carried out experimental and modeling work on fouling to develop a methodology by which the thermal conductivity of the ash deposit would be determined in a way similar to the fouling process prevailing in real systems. For that we used tubes identical in material, diameter and temperature to those used in many utility boilers. In the experimental work we placed a tube in an axially symmetric 50 kW furnace, and tested fouling from three coals, bituminous and sub-bituminous. We also developed a dynamic model for the prediction of the ash deposition growth and its heat resistance. Comparison of the model prediction and experimental results yielded satisfactory fit. Consequently, thermal resistance of heat exchanger tuber with ash deposit of those coals was determined.Copyright

Slagging of Ash From Sub-Bituminous and Bituminous Coals in a Test Combustion Facility

Slagging caused by deposit of molten fly ash on hot walls is a major concern in the operation of full-scale utility boilers. We carried out a comprehensive study, experimental and modeling, on slagging with various coals. Coal samples were taken prior and during combustion and analyzed by SEM (Scanning Electron Microscope). From the SEM analysis the coals could be divided into two types: (1) Coal with tiny particulates of the mineral matter deposited loosely on the surface of coal particles or between carbon particles (external ash) and (2) coal with the mineral matter encapsulated within the coal particles (internal ash). We found different slagging and char combustion characteristics directly related to the two coal types. It was observed that internal ash coals show higher slagging propensity and higher carbon content in the fly ash. Previous models to predict slagging did not distinguish between the two coal types and their impact on slagging and combustion behavior. We developed a model for high temperature ash deposition on the furnace walls for these two coal types. In this model, char combustion and carbon content in the fly ash are also considered. Comparison of experimental observation with calculation results from the ash deposition model show good agreement. Another conclusion from the model is that slagging propensity for internal ash coals increases with coal particle size. However, this conclusion has to be verified experimentally.© 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

Selection of Biomass Feedstock for Production of Biocoal for Coal-Fired Boilers

PGE in collaboration with EBC and MTU is carrying out a testing program to fire up to 100% of biocoal (torrefied biomass) in its 600 MW Boardman boiler. An important aspect of this program is the selection of suitable biomass feedstock from which biocoal will be produced, emphasizing potential problems of fouling and slagging in the boiler. We thoroughly tested seven different types of feedstock: Arundo Donax (AD), wheat waste, corn waste, woody hybrid poplar, and bark from hybrid poplar, woody pine, and bark from pine. It was found that all these material comprised significant amounts of soil (varying from 5–25% in weight) with low fusion temperatures and therefore must be avoided from flowing into the boiler. We developed a separation technology of the soil from the biomass and were able to obtain biomass feedstock only with the plant minerals. All separated biomass feedstock, from soil, showed mineral content that is respective to soil they grew at. Samples were characterized for ultimate and proximate analysis, ash content and analysis and fusion temperatures. AD, wheat, and corn showed high content of potassium and low flow temperatures and therefore may not be used at 100% firing test unless some of the mineral contents are removed to protect the boiler from corrosion and slagging. Woody and bark hybrid poplar were found to have high fusion temperatures; woody and bark pine showed flow temperatures around 2500°F. All four feedstock types can be used for 100% firing test, however, the ones which is mostly recommended are woody and bark hybrid poplar.Copyright

Firing Tests of Biocoal

As part of PGE-EBC-MTU collaboration of the testing program to fire up to 100% of biocoal in the 600 MW Boardman boiler we produced samples from the seven biomass feedstock: Arundo Donax (AD), wheat waste, corn waste, woody hybrid poplar, and bark from hybrid poplar, woody pine, and bark from pine. The various samples of biocoal were tested in a combustion chamber with the following results: (1) Biocoal was fired and burned providing temperature and gas concentration profiles similar to coal. (2) NOx emission from all biocoal originating from any type of biomass feedstock was found to be significantly lower than that from coal burning. (3) SOx emissions was found to correlate directly to sulfur content in the plant minerals, which is very small for all types of biomass tested. (4) Fouling was quite low for all biocoal tested, such that it can be handled with an optimized water cannons procedure. (5) Minerals in the biocoal were found to segregate from the carbon particles which means that slagging propensity can be predicted by the common slagging indices. (6) Carbon cycle analysis revealed significant reduction of CO2 when using these biomass feedstock types, particularly the bark types.Copyright

Production and Characterization of Biocoal for Coal-Fired Boilers

As part of PGE-EBC-MTU collaboration of the testing program to fire up to 100% of biocoal in the 600 MW Boardman boiler we produced samples from the seven biomass feedstock: Arundo Donax (AD), wheat waste, corn waste, woody hybrid poplar, and bark from hybrid poplar, woody pine, and bark from pine. The idea was to produce a few thousand tons of biocoal from woody and bark poplar for a 100% firing tests and from the other types to produce a 1000 tons of biocoal from each material that will be co-fired up to 10% with Powder River Basin coal. Biocoal is produced by a torrefaction which is a thermal process carried out in absence of oxygen. We have produced biocoal samples from the above biomass feedstock in two pilot facilities, one in Israel and another in Michigan. The torrefaction process comprises the following steps: (1) shredding and soil separation, (2) drying, (3) torrefaction, and (4) compaction to produce biocoal briquettes. Biocoal briquettes are essential for logistics, safety, operational, and economic considerations. The briquettes must be durable, water resistant and can be pulverized in common coal mills. The briquettes that we produced did indeed conform to these properties. A real operational challenge was working with absence of oxygen which essential for the torrefaction process as well as for safety considerations because the entire process occurs at elevated temperatures which biocoal can burn. A 30,000 t/year torrefaction facility has been constructed at the Boardman Plant site to produce biocoal required for the firing tests.© 2014 ASME