Kevin Davis

Evolution of char chemistry, crystallinity, and ultrafine structure during pulverized-coal combustion☆☆☆

Abstract The carbonaceous structure of partially reacted char samples, generated by direct injection of pulverized coal into a laboratory entrained flow reactor, was characterized by four techniques: elemental analysis, carbon dioxide vapor adsorption, x-ray diffraction, and fringe-imaging using high-resolution transmission electron microscopy. It is observed that the early stages of heterogeneous oxidation proceed in parallel with the latter stages of carbonization, leading to preferential loss of hydrogen, a reduction in surface area, and the development of crystalline order. Typical combustion times and peak temperatures are insufficient to bring about true (three dimensional) graphitization for most coals, but rather, lead to the growth of regions of turbostratic order. This ordering is seen to occur over a time scale comparable to the combustion process itself—here, on the order of 100 ms at particle temperatures of 1800 K and oxygen concentrations of 12 mol%. This work presents evidence that the reactivity of chars in the latter stages of burnout, which is critically important to the explanation and prediction of unburned carbon in flyash, is significantly impacted by the evolution of the carbonaceous matrix. Although significant evolution of internal surface area and hydrogen content (indicative of aromatic ring coalescence) occurs during early char combustion, these two phenomena do not play a major role in the char deactivation noted in previous investigations. Among the four indicators of carbon structure evaluated herein ( H C ratio, carbon dioxide surface area, crystallite dimensions by x-ray diffraction, and HRTEM images), the volume fraction of ordered material as determined by HRTEM fringe-imaging correlates best with the observed reactivity loss for Illinois #6 coal chars.

Residual carbon from pulverized-coal-fired boilers. 2. Morphology and physicochemical properties

The morphology and bulk physicochemical properties of residual carbon in eight fly ash samples from commercial power plants were investigated. Enriched carbon samples extracted from the bulk fly ash were characterized by high-depth-of-field optical microscopy, reflected-light microscopy, scanning electron microscopy, elemental analysis (C, H, O), and CO2 adsorption. The crystalline structure of the carbon was characterized by X-ray diffraction, optical reflectance, and high-resolution transmission electron microscopy fringe imaging. The results were compared with measurements on laboratory-generated chars in the early-to-intermediate stages of combustion. Compared with those chars, the residual carbon is of similar elemental composition, petrographic composition and surface area but higher crystallinity. The fuel-related mechanisms that can contribute to carbon carryover in boilers are discussed, including inertinite persistence, mineral matter encapsulation and char deactivation by pregraphitization, as well as the implications for utilization of residual carbon.

Single-particle combustion of two biomass chars

In order to assess the combustion reactivities of chars produced from the pyrolysis of woody and herbaceous biomass, we have subjected particles of Southern pine and switchgrass chars to two sets of combustion experiments. In the first, a dilute stream of biomass char particles (nominal size, 75–106 μm) is burned in a laminar flow reactor at 12 mole % O 2 and a mean gas temperature of ∼1600 K. In situ optical measurements reveal that at a given residence time in the early stages of char conversion, biomass char particles burn over a much wider temperature range (∼450 K) than coal particles (∼150 K) and that the biomass char particle temperatures span the entire range of the theoretical limits (from the slowest burning inert particles to the fastest burning diffusion-controlled particles). As biomass char conversion proceeds, however, mean particle temperatures decrease, and particle temperature distributions narrow-consequences of the preferential removal of more reactive carbon as well as a number of physical and chemical transformations of the inorganic constituents of the chars (vaporization, surface migration and coalescence, and incorporation into silicate structures). Kinetic parameters for median particles taken at the early stages of char conversion indicate that biomass chars are somewhat less reactive than low-rank lignites and subbituminous coals, somewhat more reactive than high-rank low-volatile bituminous coals, and comparable in reactivity to high-volatile bituminous coals. In the second set of combustion experiments, individual biomass char particles are suspended on an inert mesh and suddenly subjected to a hot 6 mole % O 2 environment. Video images of reflected light and near-infrared emission for a number of pine and switchgrass char particles demonstrate the heterogeneity of the biomass chars in terms of both the sequence of morphological changes and the temperature-time histories of the particles as they undergo combustion.

Near-extinction and final burnout in coal combustion

The late stages of char combustion have a special technological significance, as carbon conversionsof 99% or greater are typically required for the economic operation of pulverized coal-fired boilers. In the present article, two independent optical techniques are used to investigate near-extinction and final burnout phenomena for Illinois #6 and Pittsburgh #8 bituminous coals. Captive-particle image sequences, combined with in situ optical measurements on entrained particles, provide dramatic illustration of the asymptotic nature of the char burnout process. Single-particle combustion to complete burnout is seen to comprise two distinct stages: (1) a rapid high-temperature combustion stage, consuming about 70% of the char carbon and ending with near-extinction of the heterogeneous reactions due to a loss of global particle reactivity, and (2) a final burnout stage occurring slowly and at lower temperatures. For particles containing mineral matter, the second stage can be further subdivided into (2a) late char combustion, which begins after the near-extinction event and converts carbon-rich particles to mixed particle types at a lower temperature and a slower rate, and (2b) decarburization of ash—the removal of residual carbon inclusions from inorganic (ash) frameworks in the very late stages of combustion. This latter process can be extremely slow, requiring over an order of magnitude more time than the primary rapid combustion stage. For particles with very little ash, the loss of global reactivity leading to early near-extinction is believed to be related to changes in the carbonaceous char matrix, which evolves over the course of combustion as a result of simultaneous oxidation and heat treatment. More realistic models are needed to predict the asymptotic nature of char combustion and to make accurate predictions in the range of industrial interest.

Modeling the vaporization of ash constituents in a coal-fired boiler

Emissions of fine particulate and trace toxic metals are being subjected to increasing regulation. This paper addresses how the emission of these compounds can be influenced by changes in combustion conditions, particularly those selected to minimize NOx emissions. The vaporization and condensation of refractory oxides dominate submicron aerosol formation during the combustion of bituminous coals. The vaporization of these oxides is augmented by the reduction of refractory oxides to suboxides or metals. We used available drop tube furnace data to develop and test models for the vaporization of aluminum and iron. The vaporization of alumina is found to occur primarily via reduction of the alumina to Al2O. Although other paths are recognized to be important, the vaporization of iron is approximated by a mechanism involving FeO reduction. The models for aluminum and iron vaporization, together with previously developed models for the other refractory metals, are incorporated into a computational fluid dynamics code to determine the impact of different oxidation-temperature histories in a utility boiler. The vaporization of refractory oxides is calculated for conditions corresponding to the operation of the boiler before and after retrofitting with low-NOx burners and overfire air ports. The results show that the vaporization of refractory oxides is diminished under low-NOx operating conditions and that the different oxygen-temperature histories of particles lead to significant differences in the vaporization of oxides for particles originating from different burners. The vaporization is found to occur primarily in regimes where temperature and CO concentration are high.

Trends in predicting and controlling ash vaporization in coal-fired utility boilers

Abstract The past decade has seen a dramatic increase in the use of computational fluid dynamics (CFD) in the solution of problems related to the design and operation of pulverized coal-fired utility boilers. These tools have been increasingly used to simulate the performance of utility boilers, primarily for NOx control and associated problems of unburned carbon in fly ash and in water-wall corrosion. These models are extended to calculate the emissions of submicron particles by the vaporization and condensation of ash constituents. A single particle model for the vaporization of minerals in coal was first calibrated with results for vaporization of 14 coals in a laboratory reactor. The calibrated model was then applied in the simulation of the ash vaporization in a 500-MW opposed-wall fired boiler with 12 burners on each of the front and rear walls, and for cases before and after retrofitting the boiler with burner technology to reduce NOx emissions. The simulations showed that the ash vaporization occurred primarily during short intervals along particle trajectories when the particle temperatures increased above 1800 K. The ash vaporization decreased slightly on retrofitting the boiler, and the contributions by different burners to the total amount vaporized varied widely, particularly after the low-NOx retrofit.

Percolative fragmentation and spontaneous agglomeration

Abstract Captive particle imaging experiments were performed on over 200 coal and char particles in the pulverized size range from four coals of various rank at oxygen concentration from 3–19 mol-% and at gas temperatures of about 1250 K. Despite wide variations in single-particle behavior, the data set reveals two clear trends that provide new information on the nature of char combustion. First, the low-rank coal chars are observed to maintain their high reactivity through the late stages of combustion, thus avoiding the near-extinction events and long burnout tails observed for bituminous coal chars. Secondly, percolative fragmentation in the late stages of combustion is a rare event under these conditions. Some particles reach a percolation threshold late in combustion, but typically undergo spontaneous agglomeration rather than liberation of the incipient fragments. It is concluded that percolative fragmentation behavior in the pulverized size range is determined not only by solid-phase connectivity, but also by a real competition between disruptive and cohesive forces present at the time of formation of the colloidal-sized incipient fragments.

Prediction and real-time monitoring techniques for corrosion characterisation in furnaces

Abstract Combustion modifications to minimise NOx emissions have led to the existence of reducing conditions in furnaces. As regulations demand lower NOx levels, it is possible (to a degree) to continue to address these requirements with increased levels of combustion air staging. However, in most practical situations, a number of adverse impacts prevent the application of deep combustion air staging. One of the more important limitations is the increased corrosion that can occur on wall tubes exposed to fuel rich combustion environments. Current boiler corrosion monitoring techniques rely on ultrasonic tube wall thickness measurements typically conducted over 12 to 24 month intervals during scheduled outages. Corrosion coupons are also sometimes used; typically require considerable exposure time to provide meaningful data. The major drawback of these methods is that corrosion information is obtained after the damage has been done. Management of boiler waterwall loss and system optimisation therefore requires a real-time indication of corrosion rate in susceptible regions of the furnace. This paper describes the results of a program of laboratory trials and field investigations and considers the use of an on-line technology in combination with innovative applications, also modelling and precision metrology to better manage waterwall loss in fossil fuelled boilers while minimising NOx emissions.

A CFD Model Based Evaluation of Cost Effective NOx Reduction Strategies in a Roof-Fired Unit

To meet aggressive NOx reduction requirements, a range of NOx reduction strategies are currently available for application to pulverized coal fired furnaces. Utilities must assess the benefits and drawbacks of each viable NOx control technology to develop the best strategy for unit specific NOx control that fits within the utilities’ overall compliance plan. The installation of high capital and operating cost NOx reduction technologies, such as selective catalytic reduction, is cost prohibitive on many units. Lower cost technologies, although not capable of SCR level NOx reductions, can provide a more cost-effective approach and still achieve compliance over the fleet. This paper describes how computational fluid dynamic (CFD) modeling has been utilized by an experienced group of combustion engineers to evaluate and design cost effective NOx reduction strategies applied to a relatively unique PC fired unit, a B&W 150 MW roof-fired furnace. The unit fires bituminous coal through 10 multi-tip burners and is equipped with 10 NOx ports located below the burners. A baseline CFD model was first constructed and the predicted model results were compared with available data including NOx and CO emissions, as well as unburned carbon in fly ash. Upon completion of the baseline model, combustion alterations, including deeper staging, were evaluated. Specific burner adjustments were evaluated to allow for the deeper staging without significantly increasing unburned carbon in the fly ash, CO emissions, or near burner slagging. The CFD model was also utilized to evaluate the impact of water injection. AEP has previously utilized water injection to reduce peak combustion temperatures and thermal NOx formation rates in coal fired units for incremental NOx reductions. It is crucial that the NOx production zones in the downstream portion combustion field be identified, since these regions are most likely to produce NOx that will not be subsequently reduced prior to exiting the furnace. The CFD model was utilized to identify the most appropriate regions for water injection combined with the other combustion alterations. The results showed that NOx emissions could be reduced in this unit by approximately 37% from baseline full load emissions with no associated increase in unburned carbon in the fly ash or furnace exit CO. Burner alterations and water injection equipment based on the CFD model evaluation are currently being installed. Comparisons between the model predictions and the post retrofit performance will be provided.Copyright

NOx CONTROL OPTIONS AND INTEGRATION FOR US COAL FIRED BOILERS

This report summarizes the research that has been performed by Reaction Engineering International (REI) during the last three months on demonstrating and evaluating low NOx control strategies and their possible impact on boiler performance for firing US coals. The focus of our efforts during the last six months have been on: (1) Field Tests for RRI at the Conectiv BL England Station Unit No.1, a 130 MW cyclone fired boiler; (2) Extending our Computational Fluid Dynamics (CFD) based NOx model to accommodate the chemistry for Rich Reagent Injection (RRI) in cyclone fired boilers; (3) Applying the NOx model to evaluate RRI systems integrated into a boiler with Over Fired Air (OFA) and Selective Non-Catalytic Reduction (SNCR); (4) Field Tests of the REI Corrosion Probe at the Conectiv BL England Station Unit No.1; (5) Commence engineering study of ammonia adsorption mechanisms for Fly Ash; (6) Presentation of current program accomplishments and plans for future work to DoE staff members at NETL-FE (Pittsburgh); and (7) Presentation of preliminary field test results for RRI to EPRI CNCIG.