Pulverized Fuel Combustion Research Articles

Deactivation of iron particles during combustion and reduction

Iron is a promising carbon-free renewable energy carrier to replace fossil fuels. However, during a redox cycle, iron may experience deactivation due to structural changes, which can shorten the cycle lifetime. In this work, deactivation of iron particles in terms of changes in reactivity and morphology during the oxidation and reduction process was investigated, under both fixed-bed and suspension conditions. Simple thermal treatment in an inert atmosphere at 1200 °C had no influence on the reactivity of particles towards oxidation, even at long exposure resulting in mild agglomeration. In fixed-bed oxidation at 600 to 1100 °C, solid sintering was found to be rapid and severe at higher temperatures. However, the sintering showed no influence on the reactivity of particles towards H2 under slow-heating conditions in a TGA. In pulverized-fuel combustion in a drop tube reactor, the intensive combustion and heat release led to melting of particles and morphology change from irregular to spherical. As the reactor temperature was increased from 800 to 1200 °C, the particle reactivity in terms of reduction degree (fractional conversion from Fe2O3 to Fe) slightly decreased from 78 % to 72 %, possibly related to a minor loss of surface area. TGA experiments involving successive redox cycles showed a steady decline in reduction reactivity (750 °C, 2.5 % H2), slightly enhanced by agglomeration, while the oxidation degree (950 °C or 1200 °C, 21 % O2) remained constant. The present work indicates that both deactivation and loss of iron due to evaporation may serve to limit the energy release in iron combustion over extended cycles.

Comparative study of the inherent combustion reactivity of sawdust chars produced by TGA and in the drop tube furnace

Thermogravimetric analysis (TGA) has been widely used to obtain inherent combustion reactivity data for biomass fuels. However, fundamental information relating such data to systems that operate under conditions closer to those encountered in pulverized fuel (PF) combustion is relatively scarce. This study therefore, compares the combustion reactivity of sawdust devolatilized under slow heating conditions by TGA and fast heating conditions in a drop tube furnace (DTF) representative of PF combustion. The implications of devolatilization temperature, heating rate and residence time were considered. It was established that the char burnout profiles of the DTF char prepared at 1300 °C and 600 ms was similar to the 700 °C TGA char although, the DTF char showed a lower maximum rate. Scanning electron microscope (SEM) images revealed that the TGA chars retained a cell matrix structure with visible macropore network. In contrast, the DTF chars displayed swollen surfaces with distributed pore networks, resulting in higher BET surface areas. The activation energy of the DTF chars was smaller than those of TGA char, demonstrating the presence of greater internal diffusion control. Finally, it is demonstrated for the first time that the compensation effect for biomass char is considerably less than that reported previously for coal chars.

Behavior of Pb During Coal Combustion: An Overview

Despite the progress in understanding heavy metals behavior during coal combustion, mitigation of heavy metals emissions is still a tough challenge due to a complex character of this phenomenon. Several lists of potentially toxic elements have been presented; in most cases, Pb belongs to the elements with the greatest environmental and human-health concern. The review paper is focused upon the behavior of Pb during coal combustion. with particular attention paid to decreasing its emissions. It summarizes the dominant parameters affecting its redistribution among coal combustion streams. As gaseous emissions can quite easily pass through the particulate control device, attention was paid primarily to Pb distribution between condensed and volatilized phases. A crucial factor enhancing Pb volatility is the presence of organic or inorganic chlorides, which is discussed in detail, including their chlorination mechanisms and interactions with other fuel/flue gas species. Components decreasing Pb volatility and promoting the formation of condensed phases are also discussed (higher levels of moisture, Na, O2 etc.). Factors enhancing Pb volatility, as well as factors facilitating Pb retention, are discussed with the view of fluidized-bed combustion, pulverized-fuel combustion, or co-combustion of coal with wastes.

The impact of hydrothermal carbonisation on the char reactivity of biomass

Hydrothermal carbonisation (HTC) is an attractive biomass pre-treatment as it produces a coal-like fuel, can easily process wet biomass and wastes, and lowers the risk of slagging and fouling in pulverised fuel (PF) combustion boilers. One of the major factors in determining the suitability of a fuel as a coal replacement for PF combustion is matching the char reactivity and volatile matter content to that of coals, as these significantly affect heat release and flame stability. The char reactivity of wood and olive cake biocoals and their respective drop tube furnace chars have been studied using thermogravimetric analysis in comparison to other biomass fuels and high-volatile bituminous coal. It was found that HTC reduces the reactivity of biomass, and in the case of HTC of wood pellets the resulting biocoal has a char reactivity similar to that of high-volatile bituminous coal. Proximate analysis, X-ray fluorescence analysis, and textural characterisation were used to show that this effect is caused primarily by removal of catalytic alkali and alkaline earth metals. Subsequent torrefaction of the wood biocoals was performed to tailor their volatile matter content to match that of sub-bituminous and high volatile bituminous coals without major impact on char reactivity.

Mineralogy of Furnace Deposits Produced by South African Coals during Pulverized-Fuel Combustion Tests

Pilot-scale combustion tests were carried out on coals with low (13.5%) and high (39.6%) ash percentages from the Highveld Coalfield of South Africa, using the ALS-ACIRL 100–200 kW Combustion Test Facility. The mineral matter in the feed coals and also the chemistry and mineralogy of the ash deposits from different parts of the test furnace were analyzed using quantitative X-ray diffraction and X-ray fluorescence techniques, and the results were used to develop an understanding of the mineralogical processes taking place in the different parts of the pulverized-fuel combustion system. Low-temperature oxygen-plasma ashing and quantitative X-ray diffractometry showed that the mineral matter of both coals contained abundant kaolinite, lesser but still very significant proportions of quartz and carbonates (ankerite and calcite), and minor proportions of pyrite and other minerals. A small but significant proportion of nonmineral Ca also occurs in the organic matter, especially of the low-ash coal. The chemical composition of the combustion products taken from the different sampling points in the test facility (slagging panels, furnace ash, and fouling probes, etc.) was overall very similar for each coal sample. However, the percentages of the different crystalline minerals in the combustion products showed a wide range of variation at the different sampling points. Anorthite, derived from interaction between Ca and the aluminosilicate components, was formed from both coals at temperatures above 1300 °C, mostly as part of sintered aggregates that built up and became detached from the slagging panel in the highest temperature part of the combustion system. Anhydrite was formed by interaction of Ca with SO2; lime and periclase were also formed from the low-ash coal, where insufficient sulfur was available for complete sulfate development.

Effects of moisture release and radiation properties in pulverized fuel combustion: A CFD modelling study

Pulverized fuels (PF) prepared and fired in utility boilers always contain some moisture. For some fuels with high moisture contents (e.g., brown coals), the share of the evaporation enthalpy is quite significant compared to the heat released during combustion, which often needs to be reclaimed to improve the plant efficiency and is also expected to affect the combustion process. Thermal radiation is the principal mode of heat transfer in combustion. In PF furnaces, radiation consists of contribution from both participating gases and solid particles, in which gas and particle radiation properties play an important role. There are different methods or models in the literature to address fuel moisture release and radiation properties, some of which may be inappropriate and can produce misleading results. This paper compares the different methods and models and demonstrates their implementation and impacts via a computational fluid dynamics (CFD) study of a 609MWe pulverized coal-fired utility boiler. Overall speaking, it is suggested to add the free moisture in the fuel to the primary air stream while lump the bound moisture with volatiles in PF combustion modelling, although different methods for fuel moisture release may not induce distinct difference in combustion of PF with relatively low moisture content. For radiation, it has to be emphasized that particle radiation largely overwhelms gas radiation in PF combustion. The current tide of radiation research that over-focuses on gas radiation while over-simplifies particle radiation or even neglects particle radiation needs to be turned. For gaseous fuel combustion in which particle radiation is negligible, more generic model for gas radiative properties that can naturally and correctly accommodate the changes in combustion condition (e.g., oxy-fuel or air–fuel), account for the variations in CO2 and H2O concentrations in a flame, and include the impacts of other participating gases (e.g., CO and hydrocarbons) needs to be derived for combustion CFD community.

Sulfur Capture by Fly Ash in Air and Oxy-fuel Pulverized Fuel Combustion

Ash produced during oxy-fuel combustion is expected to differ from ash produced during air combustion because of the higher CO2 and SO2 atmospheres in which it is generated. For a quantitative understanding of the sulfation behavior of fly ash in oxy-fuel combustion, fly ash from three commercial Australian sub-bituminous coals was tested and decomposed under an inert atmosphere. Thermal evolved gas analysis was completed for ash produced in both air and oxy-fuel environments. Pure salts were also tested under the same conditions to allow for identification of the species in the ash that capture sulfur, along with thermodynamic modeling using FactSage 6.3. Sulfur evolved during the decomposition of air and oxy-fuel fly ash was compared to the total sulfur in the ash to close the sulfur balance. Both total sulfur captured by the ash and sulfur evolved during decomposition were higher for oxy-fuel fly ash than their air counterparts. Correlations of capture with ash chemistry are presented.

Lab-scale investigation of Middle-Bosnia coals to achieve high-efficient and clean combustion technology

This paper describes full lab-scale investigation of Middle-Bosnia coals launched to support selection an appropriate combustion technology and to support optimization of the boiler design. Tested mix of Middle-Bosnia brown coals is projected coal for new co-generation power plant Kakanj Unit 8 (300-450 MWe), EP B&H electricity utility. The basic coal blend consisting of the coals Kakanj: Breza: Zenica at approximate mass ratio of 70:20:10 is low grade brown coal with very high percentage of ash - over 40%. Testing that coal in circulated fluidized bed combustion technique, performed at Ruhr-University Bohum and Doosan Lentjes GmbH, has shown its inconveniency for fluidized bed combustion technology, primarily due to the agglomeration problems. Tests of these coals in PFC (pulverized fuel combustion) technology have been performed in referent laboratory at Faculty of Mechanical Engineering of Sarajevo University, on a lab-scale PFC furnace, to provide reliable data for further analysis. The PFC tests results are fitted well with previously obtained results of the burning similar Bosnian coal blends in the PFC dry bottom furnace technique. Combination of the coals shares, the process temperature and the air combustion distribution for the lowest NOx and SO2 emissions was found in this work, provided that combustion efficiency and CO emissions are within very strict criteria, considering specific settlement of lab-scale furnace. Sustainability assessment based on calculation economic and environmental indicators, in combination with Low Cost Planning method, is used for optimization the power plant design. The results of the full lab-scale investigation will help in selection optimal Boiler design, to achieve sustainable energy system with high-efficient and clean combustion technology applied for given coals.

Modelling potassium release and the effect of potassium chloride on deposition mechanisms for coal and biomass-fired boilers

A model, predicting the release of potassium compounds and its effect on the deposition of superheaters, has been recently developed. The model has been implemented into the three-dimensional CFD program AIOLOS. Concerning the release of potassium compound, the vaporization of the potassium and its reactions in the gas phase are considered. The influence of turbulence on chemistry is considered by using the eddy dissipation concept. Two simulations, one for a coal with high content of chlorine and the other for a coal with low content of chlorine were performed on a small-scale entrained flow reactor. The modelling results are discussed and compared with measurements. Furthermore, the effect of the released potassium chloride on the deposition mechanisms has also been considered. In order to predict the deposition rate, two major deposition mechanisms i.e. condensation and inertial impaction are considered in the model. The sticking probability is modelled based on the melting behaviour of the species involved in the deposition process. Deposit formation in a 0.5 MW semi-industrial pulverized-fuel combustion test facility is predicted considering different operating conditions. Deposition rates on the deposition probe from two kinds of biomass are compared and discussed.

A kinetic-empirical model for particle size distribution evolution during pulverised fuel combustion

Particle size is an essential parameter in pulverised fuel (PF) combustion as many of the problems or further areas of development in these systems are strongly influenced by the fuel and ash size distribution. This is particularly true for dynamic processes like pollutant formation, corrosion, erosion, slagging and fouling and the related decrease of the combustion and boiler efficiency. The evolution of particle size distribution (PSD) is a complex interaction of various competing chemical and physical transformations. Char oxidation, devolatilization and fragmentation, etc. represent first line physical and chemical transformations which can amend the particle size in the radiation zone. The evolution of the PSD represents the convolution of all of these physical and chemical transformations, operating over the entire size distribution. As a consequence, it is difficult to extract the relative importance of all competing size altering processes from the experiments. Various models such as break-up, thermal stress, shrinking core, percolation and particle-population model have been developed by incorporating numerous ash transformation mechanisms to predict the particle size evolution during the pulverised fuel combustion. The present work describes an adaptation of the numerical kinetic-based particle-population balance for predicting particle size evolution during PF combustion developed by Dunn-Rankin and Mitchell. The model is further simplified analytically and validated against experimental results. Several empirical parameters derived from the experiments are incorporated into the model. The resulting simplified PSD evolution model shows good agreement with literature and experimental results, with maximum 10% absolute standard deviation.

Combined Homo- and Heterogeneous Model for Mercury Speciation in Pulverized Fuel Combustion Flue Gases

A new model is developed to predict Hg-0, Hg+, Hg2+, and Hg-p in the post-combustion zone upstream of a particulate control device (PCD) in pulverized coal-fired power plants. The model incorporates reactions of mercury with chlorinating agents (HCl) and other gaseous species and simultaneous adsorption of oxidized mercury (HgCl2) on fly ash particles in the cooling of flue gases. The homogeneous kinetic model from the literature has been revised to understand the effect of the NO + OH + M HONO + M reaction on mercury oxidation. Because it is a pressure-dependent reaction, the choice of proper reaction rates was very critical. It was found that mercury oxidation reduces from 100 to 0

A diffusion-relaxation model of pulverized fuel combustion

A model for pulverized fuel combustion with allowance for the microstate of the fuel is proposed. It was shown that the burning of a fuel particle having a microstructure follows the laws of external (diffusion) and internal (relaxation) kinetics. The influence of the diffusion-relaxation mechanism on the burnout time of a fuel particle with a polymeric microstructure, its stressed state, and the pulverized fuel flame length was analyzed. The ranges of pulverized-fuel combustion parameters in which the flame length is determined by the fuel combustion or the gas jet mixing mechanism were distinguished.

Impact of co-combustion of petroleum coke and coal on fly ash quality: Case study of a Western Kentucky power plant

Petroleum coke has been used as a supplement or replacement for coal in pulverized-fuel combustion. At a 444-MW western Kentucky power station, the combustion of nearly 60% petroleum coke with moderate- to high-sulfur Illinois Basin coal produces fly ash with nearly 50% uncombusted petroleum coke and large amounts of V and Ni when compared to fly ash from strictly pulverized coal burns. Partitioning of the V and Ni, known from other studies to be concentrated in petroleum coke, was noted. However, the distribution of V and Ni does not directly correspond to the amount of uncombusted petroleum coke in the fly ash. Vanadium and Ni are preferentially associated with the finer, higher surface area fly ash fractions captured at lower flue gas temperatures. The presence of uncombusted petroleum coke in the fly ash doubles the amount of ash to be disposed, makes the fly ash unmarketable because of the high C content, and would lead to higher than typical (compared to other fly ashes in the region) concentrations of V and Ni in the fly ash even if the petroleum coke C could be beneficiated from the fly ash. Further studies of co-combustion ashes are necessary in order to understand their behavior in disposal.

Coal Blending with Petroleum Coke in a Pulverized-Fuel Power Plant

The current work investigates the performance of petroleum coke (PC) as a blended fuel under pulverized-fuel combustion conditions. Three full-scale combustion experiments were carried out: a pure Carboniferous, high volatile bituminous coal and two blends of this coal with different proportions of PC. The samples studied included feed fuels and blends, fly ashes, chars taken at different positions of the combustion chamber, and chars prepared in a drop tube reactor to test the performance of the individual fuels. The addition of PC led to a substantial increase in the unburned carbon of the fly ashes. The petrographic analysis of the granulometric fractions of the fuels revealed that this increase cannot be attributed to an enrichment in coke of the coarser fractions, as reported in the literature. On the contrary, the finer fraction contained slightly more coke than the raw blend. The petrographic analysis of the chars collected with the suction probe and the fly ashes showed that the two blended fuels were strongly enriched in PC-derived material, indicating a poorer combustibility compared to the high volatile bituminous coal. It is concluded that the reactivity of the blends in the later stages of combustion is related with the contents of PC-derived chars and the burnout itself.

Structure and composition of coal tars: An attempt to correlate molecular structure with increasing molecular mass

Near-burner-zone combustion rates in pulverized-fuel (PF) combustors depend primarily on the amount and the structure/composition of volatiles released from the fuel. Mathematical simulations of PF flames often assume that volatiles combustion takes place at rates similar to those of light hydrocarbons. However, tars constitute 60-70% of volatiles released by most power station coals; tar molecules are far larger and oxidize more slowly. The problem is usually ignored. This article describes the characterization of structures and compositions of molecules in a low-temperature coal tar, typical of material combusted in the near-burner zone. Several techniques have been used in combination. The procedure relies on the estimation of molecular mass distributions by size exclusion chromatography coupled with bulk structural characterization by several established techniques (e.g., 13 C-NMR, UV-fluorescence spectrometry). Sample fractionation by polarity and molecular mass have been found necessary for enhancing the resolution of the analytical tools, as well as for allowing meaningful correlations to be established between changing molecular mass ranges of successive fractions of the tar and the structural features of each fraction. The approach, described in some detail, extends the range of molecular masses amenable to examination to levels far above ceilings imposed by limitations of conventional GC, GC-MS, and probe-MS. The distribution of structures and molecular masses found in the tar suggests reaction pathways for the formation of soot during PF combustion.

Dry and Semi-Dry Methods for Removal of Ammonia from Pulverized Fuel Combustion Fly Ash

Fly ash from pulverized solid fuel combustion can become contaminated by ammonia during selective catalytic/noncatalytic NOx reduction processes and/or electrostatic precipitator conditioning, resulting in problems with the commercial handling, disposal, and utilization of ash. The present paper investigates the chemistry of a class of postcombustion ammonia removal processes that operate near room temperature and in the dry or semi-dry state. Laboratory experiments are carried out in which ammonia is liberated from fly ash through introduction of controlled amounts of water to the interparticle spaces using static humid air, flowing humid air, and/or flowing fog (water aerosol). Additional experiments explore the possibility of destroying ammonia using ozone as a low-temperature oxidant under both dry and wet conditions, alone or in combination with hydrogen peroxide.

Coal blend performance during pulverized-fuel combustion: estimation of relative reactivities by a bomb-calorimeter test

Coal blend performance during pulverized-fuel combustion: estimation of relative reactivities by a bomb-calorimeter test

Coal blend performance during pulverised-fuel combustion: estimation of relative reactivities by a bomb-calorimeter test

Blending coals as fuel for pulverised-fuel-fired power stations provides a way of minimising costs and increasing fuel flexibility. The use of blends can, however, introduce operating difficulties, such as poor ignition, carbon losses in ash, flame instability and emissions to atmosphere. A simple laboratory method is presented for assessing the changes in the reactivity of coal blends with blend composition. It is based on a commercial bomb calorimeter, with small modifications to hardware and procedures. The test is applicable to any size fraction. Two sets of coal samples and their blends, previously tested in a single-burner pilot plant and in a commercial power station, respectively, have been studied using the new bomb-calorimeter procedure. The combustion performance of the individual coals was not always consistent with performance at the larger scale. However, the technique was able to distinguish between the combustion reactivities of different blends and reproduce the trends observed in the pilot-scale burner and the full-sized power plant. This laboratory-scale experiment provides a simple means of assessing the relative combustion reactivity of coal blends, using equipment that is simple and generally available.

Coal Char Thermal Deactivation under Pulverized Fuel Combustion Conditions

To examine the potential for thermal deactivation during pulverized coal combustion a high-temperature wire-mesh reactor has been used to prepare chars at heating rates of 104 K s-1, temperatures up to 1800 °C, and hold times of 0−5 s, from two Argonne Premium coals: Pittsburgh No. 8, a high-volatile bituminous coal, and Pocahontas No. 3, a low-volatile high rank coal. Residual char reactivities to oxygen were determined using a nonisothermal TGA method. A comprehensive examination of the effect of preparation conditions on char reactivity is reported. Chars from the higher rank coal were relatively less reactive than chars from the lower rank coal prepared at lower temperatures, in line with accepted trends. At the higher temperatures (up to 1800 °C), more typical of full-scale pulverized coal combustion, the trend was reversed. Due to the greater propensity of the lower rank coal for thermal deactivation, high-temperature chars from the higher rank coal were found to be relatively more reactive than ch...

98/02896 Mineral matter transformation in pulverized-fuel combustion and entrained-flow gasification: Kiel, J. H. A. et al. VDI-Ber., 1997, 1313, 223–228

98/02896 Mineral matter transformation in pulverized-fuel combustion and entrained-flow gasification: Kiel, J. H. A. et al. VDI-Ber., 1997, 1313, 223–228