Ezra Bar-Ziv

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).

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

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

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