Pulverized Coal Furnace Research Articles

Evaluation of anthracite combustion and NOx emissions under oxygen-staging, high-temperature conditions

Oxy-coal combustion could be an alternative to reduce the high NOx emissions and unburned carbon (UBC) loss in pulverized-coal furnaces burning anthracite. Experimental evaluation of anthracite burnout and NOx emissions under various oxygen-staging conditions was performed in a high-temperature vertical tandem-type drop-tube furnace (DTF) system, in which a temperature of as high as 1650°C can be achieved. Relatively low NOx emissions and very high UBC in ash (i.e., levels of 1150mgm−3 at 6% O2 and 18.47%, respectively) were observed in an air-firing atmosphere as over fire air (OFA) ratio was 20.8% at 1600°C. NOx emissions were dramatically lower in oxy-coal combustion compared with those in an air-firing atmosphere and identical low NOx emissions (200mgMJ−1) occurred at higher stoichiometric ratio in the fuel-rich zone (SR1) with increasing oxygen concentration. High burnout efficiency can be obtained concurrently under high-concentration oxygen conditions, and the effect of oxygen staging on burnout was weakened with increasing oxygen concentration. In the fuel-rich zone, char-CO2 gasification reaction was promoted under oxygen-staging, high-temperature conditions, which contributed to the char consumption and early release of char-N. In addition, it was found that burnout efficiency was improved as the mixture of O2/CO2 was substituted by pure oxygen for OFA, which was of weak effect on the NOx generation simultaneously. Accordingly, the feasibility and availability of the high-temperature, oxygen-staging combustion technology on anthracite combustion and low NOx emissions was further discussed.

Effect of silicon–aluminum additives on ash fusion and ash mineral conversion of Xinjiang high-sodium coal

The high sodium content in Xinjiang coal ash induces severe fouling and slagging in pulverized coal furnaces. To solve these problems, many people have reported to use silicon–aluminum additives. In this paper, the effects of silicon–aluminum additives (SiO2, kaolin, and fly ash) on ash fusion characteristic, mineral transformation, and sodium emission were investigated. The results showed that the ash fusion temperatures (AFTs) manifested a pattern of decrease first and increase later along with an increasing in silicon–aluminum additive ratios. To achieve higher AFTs than that of raw coal ash, the blending proportion of silicon–aluminum additives is recommended as high as 9%. The causes for these results were explored. When the proportion of the additives was 3%, Ertixiite (Na2Si2O5), nepheline (NaAlSi3O8), lazurite (Na7Al6Si6O24S3) and hauyne (Na6Ca2Al6Si6O24(SO4)2), were formed at 1150°C. These species with low melting temperatures are flux minerals and lower ash fusion temperature, resulting in the lowest AFTs. This is the main cause of lower ash fusion point in the case of lower blending ratios. Moreover, almost all of the tested additives show the capability of capturing sodium, in which, the capturing capability of SiO2 is the best. For sodium capture capacity of the three additives under most tested conditions, the optimum temperature was around 1000°C.

Numerical investigation on the flow, combustion, and NOX emission characteristics in a 660 MWe tangential firing ultra-supercritical boiler

A three-dimensional numerical simulation was carried out to study the pulverized-coal combustion process in a tangentially fired ultra-supercritical boiler. The realizable k- ε model for gas coupled with discrete phase model for coal particles, P-1 radiation model for radiation, two-competing-rates model for devolatilization, and kinetics/diffusion-limited model for combustion process are considered. The characteristics of the flow field, particle motion, temperature distribution, species components, and NOx emissions were numerically investigated. The good agreement of the measurements and predictions implies that the applied simulation models are appropriate for modeling commercial-scale coal boilers. It is found that an ideal turbulent flow and particle trajectory can be observed in this unconventional pulverized-coal furnace. With the application of over-fire air and additional air, lean-oxygen combustion takes place near the burner sets region and higher temperature at furnace exit is acquired for better heat transfer. Within the limits of secondary air, more steady combustion process is achieved as well as the reduction of NOx. Furthermore, the influences of the secondary air, over-fire air, and additional air on the NOx emissions are obtained. The numerical results reveal that NOx formation attenuates with the decrease in the secondary air ratio ( γ2nd) and the ratio of the additional air to the over-fire air ( γAA/ γOFA) was within the limits.

CFD investigation on the flow and combustion in a 300 MWe tangentially fired pulverized-coal furnace

The characteristics of the flow, combustion and temperature in a 300 MWe tangentially fired pulverized-coal furnace are numerically studied using computational fluid dynamics. The mathematical model is based on a Eulerian description for the continuum phase and a Lagrangian description for coal particles. The combustion reaction scheme was modeled using eddy dissipation concept. The application of a proper turbulence model is mandatory to generate accurate predictions of flow and heat transfer during combustion. The current work presents a comparative study to identify the suitable turbulence model for tangentially fired furnace problem. Three turbulence models including the standard k-e model, the RNG k–e model and the Reynolds Stress model, RSM are examined. The predictions are compared with the published experimental data of Zheng et al. (Proc Combust Inst 29: 811–818, 2002). The RNG k–e model proves to be the most suitable turbulence model, offering a satisfactory prediction of the velocity, temperature and species fields. The detailed results presented in this paper may enhance the understanding of complex flow patterns and combustion processes in tangentially fired pulverized-coal furnaces.

Combustion and NOx Emission Characteristics with Respect to Staged-Air Damper Opening in a 600 MWe Down-Fired Pulverized-Coal Furnace under Deep-Air-Staging Conditions

Deep-air-staging combustion conditions, widely used in tangential-fired and wall-arranged furnaces to significantly reduce NOx emissions, are premature up to now in down-fired furnaces that are designed especially for industry firing low-volatile coals such as anthracite and lean coal. To uncover combustion and NOx emission characteristics under deep-air-staging conditions within a newly operated 600 MWe down-fired furnace and simultaneously understand the staged-air effect on the furnace performance, full-load industrial-size measurements taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash were performed at various staged-air damper openings of 10%, 20%, 30%, and 50%. Increasing the staged-air damper opening, gas temperatures along the flame travel (before the flame penetrating the staged-air zone) increased initially but then decreased, while those in the staged-air zone and the upper part of the hopper continuously decreased and increased, respectively. On opening the staged-air damper to further deepen the air-staging conditions, O2 content initially decreased but then increased in both two near-wall regions affected by secondary air and staged air, respectively, whereas CO content in both two regions initially increased but then decreased. In contrast to the conventional understanding about the effects of deep-air-staging conditions, here increasing the staged-air damper opening to deepen the air-staging conditions essentially decreased the exhaust gas temperature and carbon in fly ash and simultaneously increased both NOx emissions and boiler efficiency. In light of apparently low NOx emissions and high carbon in fly ash (i.e., 696-878 mg/m(3) at 6% O2 and 9.81-13.05%, respectively) developing in the down-fired furnace under the present deep-air-staging conditions, further adjustments such as enlarging the staged-air declination angle to prolong pulverized-coal residence times in the furnace should be considered to improve the deep-air-staging combustion configuration.

Characterization of Combustion and NOx Emissions with Respect to Overfire Air Damper Opening in a Down-Fired Pulverized-Coal Furnace

The application of deep-air-staging combustion technology to significantly reduce NOx emissions and simultaneously control combustible loss to acceptable levels is already well established in tangential-fired and wall-arranged furnaces. However, this technology has not yet been widely applied to down-fired furnaces, and thus there is a requirement to investigate the potential benefits. This work presents an experimental investigation of the combustion and NOx emission characteristics in a newly operated down-fired 600 MWe furnace equipped with a deep-air-staging combustion configuration. Full-load industrial-size measurements were performed by acquiring various data such as gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash. Overfire air (OFA) damper opening settings of 15%, 30%, 50%, and 70% were tested in turn so as to evaluate the OFA effect, following which the staged-air damper was opened from 30% to 50% to assess its ability to improve the poor burnout associated with low NOx operating conditions. It was found that, as OFA was opened, gas temperatures in the burner zone and along the flame travel path (prior to penetration of the flame into the hopper region) exhibited an initial increase and subsequently decreased. Similar to trends that have been seen in most tangential-fired and wall-arranged furnaces, opening OFA continuously increased the exhaust gas temperature, CO levels in flue gas, and carbon in fly ash, while simultaneously decreasing NOx emissions and boiler efficiency. When opening OFA to generate deep-air-staging conditions, only the 50% setting balanced NOx emissions and combustible loss, although it was still generating high levels of carbon in fly ash. With the OFA damper opening fixed at 50%, increasing the staged-air damper opening from 30% to 50% reduced combustible loss and had a slight influence on NOx emissions. On the basis of the observed high levels of carbon in fly ash, accompanied by reasonably low levels of NOx emissions [9.81% and 878 mg/Nm3 at 6% O2 (dry)], a combustion retrofit based on enlarging the shallow staged-air angle is recommended to improve furnace burnout under deep-air-staging conditions.

A New Agro/Forestry Residues Co-Firing Model in a Large Pulverized Coal Furnace: Technical and Economic Assessments

Based on the existing biomass co-firing technologies and the known innate drawbacks of dedicated biomass firing, including slagging, corrosion and the dependence on fuel, a new model of agro/forestry residue pellets/shreds and coal co-fired in a large Pulverized Coal (PC) furnace was proposed, and the corresponding technical and economic assessments were performed by co-firing testing in a 300 MW PC furnace and discounted cash flow technique. The developed model is more dependent on injection co-firing and combined with co-milling co-firing. Co-firing not only reduces CO2 emission, but also does not significantly affect the fly ash use in cement industry, construction industry and agriculture. Moreover, economic assessments show that in comparison with dedicated firing in grate furnace, agro/forestry residues and coal co-firing in a large PC furnace is highly economic. Otherwise, when the co-firing ratio was below 5 wt%, the boiler co-firing efficiency was 0.05%–0.31% higher than that of dedicated PC combustion, and boiler efficiencies were about 0.2% higher with agro/forestry residues co-firing in the bottom and top burner systems than that in a middle burner system.

Influence of forward scattering on prediction of temperature and radiation fields inside the pulverized coal furnace

A possibility of simplification of the scattering phase function of a pulverized coal flame was analyzed in the paper. It was showed that the type of radiation scattering of a pulverized coal flame is between two limiting cases: isotropic and forward scattering. A comprehensive mathematical model of a tangentially fired furnace by pulverized coal was formed. Radiative heat transfer was modeled using the six-flux model. Grid independent results of the numerical simulations were obtained. The mathematical model was verified by comparison of the results of numerical simulations with results of measurements. The influence of the type of radiation scattering on results of numerical simulation was analyzed through the relative and average differences of the gas-phase temperatures, the total radiation fluxes, and the absorbed wall fluxes of the left furnace wall. The investigation showed that the total radiation fluxes were considerably influenced by the type of radiation scattering. On the other hand, the gas-phase temperatures and the absorbed wall fluxes were much less influenced by the type of radiation scattering. The results justify the use of the scattering phase function corresponding to isotropic scattering in radiation models of comprehensive mathematical models of pulverized coal fired furnaces.

Process Evaluation and Detailed Characterization of Biomass Reburning in a Single-Burner Furnace

Reburning technology is considered to be one of the most promising and cost-effective NOx reduction strategies for coal combustion systems. Although natural gas and coal have been the most widely studied reburn fuels, use of biomass in the reburning process is also attractive because of the nearly CO2-neutral character as well as the lower NOx and SO2 emissions of biomass, with respect to fossil fuels. Further studies on process evaluation and detailed characterization of biomass reburning, especially on the semi-industrial pulverized-coal furnace, are required before large-scale biomass reburning implementation can be realized. In the present study, process evaluation for the sawdust, rice husk, and dry sludge reburning have been performed on a semi-industrial pulverized-coal single-burner furnace. The detailed O2, NO, CH4, HCN, and NH3 concentration distributions in the reburn zone have been obtained through in-furnace measurements. The effects of the reburn fuel ratio and biomass carrier gas on the NO ...

Size Distribution Functions of Ultrafine Ashes From Pulverized Coal Combustion

The major drawbacks of coal combustion are connected with the large emission of particulate matter. This paper deals with the experimental investigation of ultrafine (D < 100 nm) and nano (D < 10 nm) ashes formed during conventional high temperature pulverized coal combustion on a laboratory-scale reactor. The reactor consists of an atmospheric pressure laminar premixed flame homogeneously doped with pulverized coal particles, monodisperse in size. It allows the investigation of the early stage of ash formation. Generated aerosols, sampled using high dilution probes, are conveyed into high resolution Differential Mobility Analyzers (DMA) to perform on-line measurements of the size distribution function of ultrafine ashes. Two DMA systems are used: the first is equipped with a Faraday cup electrometer detector and identifies particles as small as 1 nm, and the second is equipped with a condensation nucleus counter and exhibits 3 nm lower detection limit. Measurements have been performed at several dilution ratios to understand the nature and behavior of detected particles. Results indicate that nanosized coal ashes are formed during pulverized coal combustion. They are the most abundant in number and also a significant fraction of ultrafine ash mass, implying they should be detected at the exhaust of pulverized coal furnaces.

The modelling of the generation of volatiles, H2 and CO, and their simultaneous diffusion controlled oxidation, in pulverized coal furnaces

The purpose of this paper is to contribute to the mathematical modelling of the combustion of coal particles in pulverized coal furnaces. The model deals with the gas and solid phases of the flow. For the coal particles a Lagrangian description is used, taking into account the simultaneous processes of moisture evaporation and devolatilization together with the heterogeneous gasification reactions of the char. An Eulerian description will be used for the distributions of temperature and concentrations in the gas phase, with the effect of the particles represented by volumetric sources of heat, mass and momentum. The gas phase oxidation reactions of the volatiles, H2 and CO will be modelled using the assumption of infinitely fast rates; the Burke–Schumann analysis will be generalized to account for the competition for oxygen of CO, H2 and the volatiles. These reactions may occur, in the form of group combustion, in a gaseous thin diffusion flame separating a region without oxygen, where the coal particles generate volatiles, H2 and CO, from a region with oxygen, where the reactions may occur inside the particles or, outside, in diffusion flames surrounding the individual particles, even though for small particle sizes the gas phase reactions can be considered as frozen near the particles. The analysis will provide relations for the volumetric sources appearing in the gas phase description, and for the rates that determine the evolution of the temperature and mass content of moisture, volatiles and char in the particles.

Computational fluid dynamics technique as a tool for description of the phenomena occurring in pulverized coal combustion systems

The paper presents the overall frame, principal steps, and some results of a numerical model of a power boiler furnace that uses pulverized coal, with tangential disposition of the burners. This model demonstrates the application potential of the computational fluid dynamics (CFD) technique and of the computational thermal analysis. Complex three-dimensional furnace geometry, two-phase turbulent flow, coal combustion, and heat transfer have been examined. Two numerical modelling approaches were employed in the investigation, both based on the Euler-Lagrangean two-phase flow concept and on the gas-phase semiempirical k-ɛ turbulence model. The first approach is based on a specially developed comprehensive model of processes in a pulverized coal furnace. In the second case, a commercial CFD code is used to create a three-dimensional furnace model. Some distinctive results concerning the performance of the boiler that was examined are presented graphically. On the basis of a comparison between the simulation predictions and available site measurements, a conclusion can be drawn that these two numerical codes provide realistic insight into furnace processes. Qualitative agreement indicates that the calculations are reasonable and validates the submodels employed.

Computation of 2-D pinhole image-formation process of large-scale furnaces using the discrete ordinates method

In most of the discrete ordinate schemes (DOS) reported in the literature, the discrete directions are fixed, and unable to be arbitrarily adjusted; therefore, it is difficult to employ these schemes to calculate the radiative energy image-formation of pulverized-coal furnaces. On the basis of a new DOS, named the discrete ordinate scheme with (an) infinitely small weight(s), which was recently proposed by the authors, a novel algorithm for computing the pinhole image-formation process is developed in this work. The performance of this algorithm is tested, and is found to be also suitable for parallel computation.

Applying Eulerian and Lagrangian Approaches to the Modeling of Dry Desulfurization Process in Pulverized Coal Furnaces

The dry additive desulfurization process (DAP) is an appropriate approach for SO X -reduction in power plants with small and medium capacity and for the retrofit of existing boilers with spatial limitations. In engineering practice, it can be optimized by means of numerical modeling. In the present work, a comprehensive model for simulating the DAP in pulverized coal furnaces is established. A pore model (PM)3069 is suggested to describe the gas-solid reaction between the sorbent and SO X . The overall DAP model is implemented into a coal combustion simulation CFD program AIOLOS, because of the feature of two-phase-flow of DAP, in both Eulerian and Lagrangian (particle tracking) schemes. DAP-desulfurization in engineering processes can be simulated. Numerical studies were performed through various calculations with both Eulerian and Lagrangian approaches for a small-scale furnace. The behaviors and results of the two approaches were compared. The present DAP model offers reasonable results. In addition, it was found: (a) For sorbent particles finer than 60-70 μm, quite similar desulfurization results can be obtained from the two modeling schemes. Above this size range the slip between phases shows significant influence. (b) The CPU-time of the Lagrangian approach increases rapidly with decreasing particle size below about 30 μm. In the present study with 400 tracing particles, the Lagrangian approach requires less computing time than Eulerian above particle size of 60-70 μm. (c) The Lagrangian approach is more suitable for handling two-phase flow with detailed particle size distributions. The Eulerian approach has advantages for treating flows with finer particle suspensions. (d) The Eulerian scheme has less convergence problems, whereas the Lagrangian approach is more sensitive to the computation procedure and requires small relaxation factors.

Radiative Properties of Residual Char and Its Effects on Radiative Heat Transfer in Pulverized Coal Fired Furnaces

AbstractThis paper gives a comprehensive modelling analysis of the radiative properties of residual char and its effect on radiative heat transfer in pulverised coal furnaces. Firstly, the effective optical constant is introduced into the model of the radiative properties of unburned char. Three structure models of unburned char are considered, i.e. constant diameter model, shrinking core model, and a transient model. The result shows that regardless of the structure of residual char, the absorption in the initial stage of combustion of large char particles (D&gt;10μm) does not decrease, while a sharp decrease occurs near the burnout (B&gt;0.9). Then radiative property models of particles were incorporated into a CFD code to investigate its effect on overall radiative heat transfer in pulverized coal furnaces. The numerical results of a 300 MW tangentially fired furnace show that residual char shields radiative heat transfer along the radial direction in the upper zone of the furnace, and thus the temperature at the exit of the furnace is higher. By ignoring the additional absorption of residual char, it will lead to a lower predicted radiant heat flux to the area of platen superheater

3-D numerical model for predicting NO x emissions from an industrial pulverized coal combustor

The paper describes a three-dimensional computer simulation developed to predict the behaviour of industrial pulverized coal furnaces. The model was applied to a 300 MWe front-wall fired boiler under different operating conditions. The main characteristics of the current model are the use of the Lagrangian framework to describe the particulate phase, the discrete transfer model to handle the radiation transmission, and the standard k− ε model for turbulence treatment. The turbulent dispersion of particles is described by a new stochastic self-stable strategy. The NO x formation mechanism is calculated in a post-processor routine. Measurements of NO x emissions were made in the flue gases; the results agree well with those obtained by the simulations.

Characterization of Synthetic‐Coal Char Particles using fractal dimension analysis

AbstractThe development and change of surface ruggedness in chars was studied at conditions typical in a pulverized coal furnace. The fractal dimension, a measure of surface ruggedness, of chars was measured using physisorption techniques. By adjusting the temperature encountered (1173 to 1773 K) and residence time (0.1 to 1.5 s) of the synthetic coal (sized to 46–106 μm diameter), chars at different stages of combustion were prepared in a laminar flow (drop‐tube) furnace. The particles were quickly cooled and quenched in an inert atmosphere. The samples were examined using a scanning electron microprobe, and their fractal dimensions were determined using gas physisorption. The adsorption data were used to test if the char surface was fractal on a molecular scale, to determine the fractal dimension, and to quantify changes in the fractal dimension during combustion. The fractal dimension of the unburned synthetic coal was approximately 2. The fractal dimension increased as high as 2.85 as the carbon matrix burned away and exposed mineral moieties. However, as combustion continued the carbon burned completely away leaving a mineral fly ash particle with a fractal dimension as low as 2.47.

Interaction of sodium, sulfur, and silica during coal combustion

The interaction of sodium, sulfur, and silica at conditions typical in a pulverized coal furnace was investigated by using both model mixtures and a synthetic coal. The model mixtures consisted of selected inorganic constituents that were well mixed in proportions typically found in low-rank coal. The synthetic coal consisted of a furfuryl alcohol polymer with appropriate amounts of sodium, sulfur, and silica to duplicate the characteristics of low-rank coal

Pulverized-coal flame stability: effect of the burner quarl cooling

This paper presents flame stability data for a pulverized-coal furnace having a water-cooled quarl and burning U.K. Gedling coal. The water-cooled quarl has the same geometry as that of a conventional refractory quarl which was the subject of a previous study. The influences of swirl number, excess air, momentum ratio and burner load on flame stability have been studied. It was observed that flame stability generally improved at high swirl numbers, high excess air levels and low ratios of primary to secondary momenta. The stability performance of the water-cooled quarl was significantly inferior to that of the corresponding refractory quarl. However, the elimination of the quarl slagging represents an important advantage of the water-cooled quarl.

Radiative heat-transfer model in the interior of a pulverized coal furnace

A practical mathematical model simulating radiative heat transfer in the furnace of a pulverized coal boiler is presented. The inclusion of this model in a pulverized coal combustion model allows for testing its validity and its sensitivity to furnace walls and particle emissivity values, by comparison with measurements in a 550 MW power plant boiler.