Pulverized Coal Furnace Research Articles

Improved method for calculating fuel burnout in pulverized coal furnaces

An improved method of engineering calculation of the degree of burnout of pulverized solid organic fuel in the flare furnaces of steam boilers is proposed, taking into account the kinetics of the processes of moisture evaporation, the release of volatile substances with a separate assessment of individual gaseous components and tar release and their combustion, secondary cracking of tars, combustion and gasification of coke residue, conversion of mineral part of the fuel, which allows for assessment and control of the implementation of material balances of individual stages, stages and, in general, the entire gross process of coal combustion, and ultimately an adequate determination of the magnitude of heat losses with mechanical and chemical incompleteness of combustion. Methodological provisions have been developed to take into account the kinetic and diffusion processes of the thermochemical transformation of coal to establish a quantitative relationship between the temperature-time characteristics of the combustion of pulverized coal particles and the local parameters of the combustion process. The technique is implemented in the form of specialized algorithmic and software and makes it possible to assess the degree of fuel burnout, both for the active combustion zone and for zones located in the cooling chamber of the combustion space during design and operational adjustment work.

Experimental study on the effects of co-firing mode and air staging on the ultra-low load combustion assisted by water electrolysis gas (HHO) in a pulverized coal furnace

Developing zero-carbon fuel (H2/NH3) co-firing technology with pulverized coal can improve the low-load flame instability and pollutant emissions of boilers during peak shaving. In this study, we propose to assist the low-load combustion of coal powder furnaces with the safer water electrolysis gas (HHO). To further optimize the combustion strategy, a one-dimensional furnace combustion system coupled with an HHO gas generation and transportation system was used to investigate the effects of injection methods and air staging on the flue gas emission and auxiliary combustion characteristics of the lignite load reduction process and ultra-low load. The results indicate that reducing the coal combustion load achieves carbon reduction and reduces actual CO2 emissions. The excess air coefficient increases, resulting in higher NOX and lower CO emissions. Air staging can control NOX and CO emissions during load shedding, with a 40.49 % reduction in NOX at 30 % load. Under ultra-low load, HHO-assisted combustion increases the oxygen concentration in the furnace, increasing NOX emissions, while SO2 decreases and then increases. However, the effect of HHO gas premixed mode (PM) on NOX generation is weaker than that of staged mode (SM). As the flow rate of HHO increases, HHO-SM promotes the conversion of CO to CO2 and reduces CO emissions, while CO emissions under PM remain at ∼10 ppm. Both HHO injection methods exhibit assisted combustion effects for ultra-low load operation. Due to the different effects of the two on the recirculation zone inside the combustion, the auxiliary combustion effect of PM is superior than that of SM. At 1800L/h HHO, the decrease in combustion instability coefficient (βT) of PM is 57.14 %, higher than that of SM. Air staging is beneficial for stable combustion under ultra-low load, but it can affect the auxiliary combustion of HHO gas. Under ultra-low load HHO co-firing conditions, 11%-OFA can also control NOX and CO emissions.

Retrofit of a 600 MW Down-Fired Pulverized-Coal Furnace for Low NOx Emission

Aiming at solving the problem of high NOx emissions of a down-fired boiler, a new combustion system has been proposed by means of the numerical simulation using Ansys Fluent. The coal-lean stream (tertiary air), which was originally mixed with a separated overfired air (SOFA) stream on the front and rear walls of the upper furnace, was relocated to the lower zone of the furnace after retrofit. The secondary-air slots were transformed into a new annular port type, which was injected into the furnace with a down-tilt angle to increase the residence time of the coal stream. Furthermore, the effect of secondary air distribution and velocity of coal stream on performance was studied. After retrofitting the combustion system, the NO emissions were effectively controlled, decreasing from 906 mg Nm−3 to 576 mg Nm−3, but the carbon content of fly ash increased from 2.46% to 5.78%. Aiming at decreasing the carbon content of fly ash, the effect of coal/primary air velocity on the arch was studied. Less carbon content in fly ash can be observed for the lower arch airflow velocity. The results showed that the NO emissions can be controlled below 595 mg Nm−3, and the carbon content of fly ash was reduced to 3.39% when the velocity was decreased to 18 m s−1.

Distributions of and environmental risks posed by Cr and Zn when co-treating solid waste in different kilns

In order to dispose solid waste reasonably and provide reference data for solid waste co-treatment in industrial kilns, coal chemical products were co-treated in a pulverized coal furnace and refuse-derived fuel was co-treated in a gasifier-coupled pulverized coal furnace system. The distribution and environmental risks of Cr and Zn in different kilns were compared and analyzed. The Cr and Zn distributions in the solid products from the pulverized coal furnace tests were similar. Fly ash contained > 80% of the Cr and Zn. In the gasifier, cyclone dust and gasification gas contained only ∼ 60% of the Cr and Zn, and gasification slag contained > 40% of the Cr and Zn. The gasification gas contained ∼ 33% of the Cr and Zn volatilized. The pulverized coal furnace temperature was > 1,500 °C. Most of the Cr and Zn volatilized and then condensed, so became enriched in the fly ash. The gasifier temperature was ∼ 750 °C, so less volatilization occurred and Cr and Zn became enriched in the gasification slag. The Cr and Zn concentrations in leachates of the solid products were lower than the limits of “GB 5085.3–2007”. However, the Cr and Zn concentrations in the gasification slag and cyclone dust leachates were close to the limits and tens to hundreds of times higher than the concentrations in the pulverized coal furnace fly ash and slag leachates. The low temperatures and low-oxygen environments of gasifiers are not conducive to heavy metals being stable in the solid products, and the environmental risks posed by heavy metals in the solid products are high. The risks to the environment are less serious for co-treating solid waste in pulverized coal furnaces than gasifiers.

Determination of the sootblower activation moment for biomass co-firing in a pulverized coal furnace

The pulverized coal-fired furnaces are expected to use co-firing with biomass for environmental reasons. Although the non-uniform ash deposits are formed on the furnace walls, the uniform deposits could be used for the analysis of the furnace operation. The objective of this investigation was determination of the uniform deposit thickness, used as a criterion for prediction of the sootblower activation moment in coal-biomass co-firing. The investigation comprised numerical simulations for uniform and non-uniform deposits to find the relative differences for the selected variables that were important for the sootblower activation: the mean wall fluxes and flame temperatures. The local thicknesses of the non-uniform deposits were determined by the gamma distribution for several mean and standard deviation values using the inversion method. The thicknesses of the uniform deposits were considered among the measures of central tendency: mode, mean, and median, of the non-uniform deposits. The mean was expected to provide the smallest relative differences, while the mode was excluded from further consideration after analysis of its values. The median was found to be better choice than the mean, as it provided smaller relative differences of the selected variables for the thick deposits, which were important for the sootblower activation. The method based on comparison of the uniform deposits for coal firing and those for the co-firing with biomass was proposed for the prediction of the sootblower activation moment. The method can be used for the selection of the operational regimes for coal-biomass co-firing.

Rare-Earth Elements Extraction from Low-Alkali Desilicated Coal Fly Ash by (NH4)2SO4 + H2SO4.

Coal fly ash (CFA) obtained from pulverized coal furnaces is a highly refractory waste that can be used for alumina and rare-earth elements (REEs) extraction. The REEs in this type of CFA are associated with a mullite and amorphous glassy mass that forms a core-shell structure. In this research, it was shown that complete dissolution of amorphous aluminosilicates from the mullite surface with the formation of the low-alkali mullite concentrate prior to sulfuric acid leaching with the addition of (NH4)2SO4 helps to accelerate the extraction of REEs. The extraction degree of Sc and other REEs reaches 70-80% after 5 h of leaching at 110 °C and acid concentration of 5 M versus less than 20% for the raw CFA at the same conditions. To study the leaching kinetics of the process, the effects of temperature (90-110 °C), liquid-to-solid ratio (5-10), and leaching time (15-120 min) on the degrees of Al and rare-earth elements (REEs) extraction were evaluated. After 120 min of leaching at 110 °C and L/S ratio = 10, the extraction of Al was found to be lower than 30%. At the same time, total REEs (TREE) and Fe extraction were greater than 60%, which indicates that a part of the TREE was transferred into the acid soluble phase. After leaching, the residues were studied by laser diffraction (LD), X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM-EDS) to evaluate the leaching mechanism and the solubility of Al- and Fe-containing minerals, such as mullite, hematite, and amorphous aluminosilicate.

Effect of wet flue gas desulfurization on the concentrations and component profiles of condensable particulate matter from ultralow emission coal-fired power plants

This study investigated effect of wet flue gas desulfurization (WFGD) on the concentrations and component profiles of condensable particulate matter (CPM). This research was carried out in two ultralow emission coal-fired power plants (CFPPs), which were equipped with pulverized coal furnaces (PC), in Hunan province, China. The CPM concentrations were measured following the US EPA Method 202. The stack emissions of filterable particulate matter (FPM) met the Chinese ultralow emission standards, and the CPM concentrations from CFPP A and B were respectively 12.12 and 17.64 mg/Nm3. In the WFGD system, the removal efficiencies of the FPM and CPM were respectively 80% and 91% in CFPP A, 83% and 81% in CFPP B. SO42− accounted for the largest fraction of the total detected water-soluble ions in the CPM. Alkanes were the main component of the organic fraction in the CPM, which accounted for 95% in CFPP A and 92% in CFPP B. The removal efficiency of inorganic components from the CPM in CFPP A and B were 81% and 92%, respectively, possibly because the water-soluble ions could be dissolved and removed in the slurry; however, due to the limited solubility of alkanes and esters, the removal efficiency of organic components from the WFGD in CFPP A and B were only 17% and 31%, respectively. The emission factors (EFs) of the CPM for CFPP A and B range from 0.090 to 0.979 kg/(t of coal).

Discussion on Coking in Pulverized Coal Furnace Combustion Process of Thermal Power Plant

Discussion on Coking in Pulverized Coal Furnace Combustion Process of Thermal Power Plant

Co-combustion characteristics and kinetics of meager coal and spent cathode carbon block by TG-MS analysis

As a hazardous waste, the spent cathode carbon block (SCCB) has a high calorific value while it is difficult to fire, its harmless disposal is a major difficulty at present. Herein, a method of mixed combustion of meager coal and SCCB in a pulverized coal furnace for disposal of SCCB is proposed, and thermogravimetric mass spectrometry (TG-MS) is used to characterize the combustion and gas release characteristics. The effects of the heating rate and mixing ratio on combustion are analyzed as well. The result shows that the comprehensive combustibility index and combustion stability index of SCCB-5 at a heating rate of 50 °C/min are both the highest. Abundant oxygen-containing groups in SCCB promote the co-combustion process. The release of hydrogen fluoride is relatively low below 1000 °C so that the use of pulverized coal boilers meets the temperature requirement for disposal of SCCB. The good melting characteristics of ash after mixing sintering also confirm this point. Finally, the kinetic calculation results show that the combustion activation energy is the lowest when the mixing ratio is 5%, which is in good agreement with the experimental results. The highest activation energy values for the combustion of meager coal, SCCB and SCCB-5 are 46.90, 89.39, 59.87 kJ mol−1, respectively.

Numerical Simulation of Combustion in 35 t/h Industrial Pulverized Coal Furnace with Burners Arranged on Front Wall

Coal-fired industrial boilers should operate across a wide range of loads and with a higher reduction of pollutant emission in China. In order to achieve these tasks, a physical model including two swirling burners on the front wall and boiler furnace was established for a 35 t/h pulverized coal-fired boiler. Based on Computational Fluid Dynamics (CFD) theory and the commercial software ANSYS Fluent, mathematical modeling was used to simulate the flow and combustion processes under 75% and 60% load operating conditions. The combustion characteristics in the furnace were obtained. The flue gas temperature simulation results were in good agreement with experimental data. The simulation results showed that there was a critical distance L along the direction of the furnace depth (x) and Hc along the direction of the furnace height (y) on the burner axis. When x < L, the concentration of NO decreased sharply as the height increased. When y < Hc, the NO concentration decreased sharply with an increase in the y coordinate, while increasing dramatically with an area-weighted average gas temperature increase in the swirl combustion zone. This study provides a basis for optimizing the operation of nitrogen-reducing combustion and the improvement of burner structures.

Combustibility and Cofiring of Coal Gasification Fine Ash with High Carbon Content in a Full-scale Pulverized Coal Furnace

Coal gasification fine ash (CGFA) having high carbon content is produced in large quantity as a by-product from coal gasifiers in China. CGFA is usually disposed of by landfill as a solid waste, wh...

High Temperature Corrosion Behaviors of 20G Steel, Hastelloy C22 Alloy and C22 Laser Coating under Reducing Atmosphere with H2S

High-temperature corrosion behaviors of 20G steel, Hastelloy C22 alloy and C22 laser coating was evaluated by corrosion mass gain measurements at 450 °C. The corrosive atmosphere is 0.2 vol% H2S–0.1 vol% O2–N2, which simulated the severe high-temperature corrosion environment occurred under low-NOx combustion in pulverized-coal furnaces. Experimental results showed that the corrosion resistance of the C22 laser coating and the C22 alloy was obviously better than 20G steel. Furthermore, it should be noted that the C22 laser coating fabricated in this study displayed a higher corrosion resistance than the commercial C22 alloy although they had the same chemical composition. The severe pitting corrosion was observed in 20G steel with the corrosion products consisting of FeS2, Fe2O3 and Fe3O4. The C22 alloy and C22 laser coating exhibited the uniform corrosion and their main corrosion products were NiS2, CrS and a small amount of chromium and manganese oxides.

Research on Dynamic Characteristics of Furnace in 670 t/h Ultra-high Pressure Pulverized Coal Furnace after Burner Transformation

The power field characteristics of a 670t/h boiler in a power plant were studied after the burner transformation. The results show that the size of the aerodynamic fluid in the hearth of the burner is appropriate, the furnace is full, and the center of the strong wind ring is basically in the center of the hearth. The wind speed of the wall around the water-cooled wall is within a reasonable range; there is a certain wind speed deviation on the side, and the angle of the SOFA reverse wind can be adjusted appropriately to eliminate the residual rotation existing at the furnace outlet. The research results can provide a reference for the transformation of burners of the same type.

Research on formation mechanism of typical low-temperature fouling layers in coal-fired boilers

Comparative field experiments of typical low-temperature fouling layers are carried out in pulverized coal furnace (PCF) and circulating fluidized bed boiler (CFB). The reduction of heat transfer characteristics of test pipe is determined by the transformation of fouling layer, caused by the quantity of condensed acid solution exceeding the adsorption capacity of deposited ash particles. Based on the heat transfer performance of fouling layer covering the test pipe, as well as the appearance characteristics and degree of adhesion to the heating surface, three typical fouling layers are collected, i.e. dry loose ash deposits (85 °C), adhering ash deposits (65 °C) and viscous ash deposits (45 °C). Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) are used to analyze the formation mechanisms of three typical fouling layers. The characteristic fouling layer is determined by the reaction degree of condensed acid solution and ash particles deposited on the heating surface. Furthermore, the formation mechanisms of unsaturated acid-ash action fouling layer, supersaturated acid-ash action fouling layer and mixed acid-ash action fouling layer are proposed, providing useful guidance for the safe and efficient operation of heat exchange equipment in coal-fired boilers.

Effect of Slot Wall Jet on Combustion Process in a 660 MW Opposed Wall Fired Pulverized Coal Boiler

Abstract Numerical investigations of an anti-corrosion design and the combustion process (original conditions and optimal conditions) were conducted for a 660 MW opposed wall fired boiler. In order to solve high-temperature corrosion of the side wall, a scheme was proposed: slotting in the side wall and introducing air (closing-to-wall air) from the secondary air. The effect of anti-corrosion was disclosed in detail by varying the structures of slotting, gas velocities from nozzles and jet inclination angles. The temperature and NOx distribution in the furnace at optimized conditions were compared with those at the original operating conditions. Simulation results showed that the structures of the slot and gas velocities from the nozzles had a marked effect on anti-corrosion of the side wall. When the gas velocity was 4 m/s, an inclination angle of the gas velocity was not conducive to anti-corrosion of the side wall. When the gas velocity increased at the middle and bottom of the side wall, the anti-corrosion effect increased significantly. When the optimal scheme was adopted, the corrosion area of the side wall decreased obviously, but the furnace temperature and the NOx emission increased slightly. The detailed results of this work promote a full understanding of closing-to-wall air and could help to reduce the corrosive area in pulverized-coal furnaces or boilers.

USABILITY OF PCF-ASH AS LIGHTWEIGHT AGGREGATE IN FOAM CONCRETE

Foam concretes can be produced with aggregate or without aggregate. In this study, the possibility of using PC ash as lightweight aggregate in the production of foam concrete was investigated. Waste- PC ash is emerging from the pulverized coal furnace about 10 tons/day in a textile factory located in the vicinity of Dinar (Afyonkarahisar). The chemical properties, particle size distribution and grain densities of PC-ash were determined. Foam concrete with a dry density of 450 kg/m3 at a cement dosage of 250 kg/m3 and w/c:0.65 was produced after the aggregate analysis. The PC-ash which is used in foam concrete mix is between 0 and 200 kg/m3. 100 mm cube and 300x300x50 mm prism samples were prepared with this foam concrete. The compressive strengths of the cube samples and the thermal conductivity coefficient of the prism samples were tested at the 28th day. The average compressive strength of foam concrete samples with a density of 452 kg/m3 was determined as 1.07 MPa and the thermal conductivity coefficient was determined as 0.097 W/mK. As a result, it has been determined that PC-ash as lightweight aggregate can be successfully used in production of precast element and light floor-screed.

Radiative Properties of Coal Ash Deposits with Sintering Effects

The heat-transfer characteristics in the fireside of a pulverized-coal furnace are affected, among other factors, by the physical and chemical characteristics of the ash deposits. Indeed, the physical state of the ash deposits and their chemical composition determine the radiative and conductive properties of the furnace walls. Particularly, several complex mechanisms are involved in the radiative heat-transfer process at the walls that restricts the absorption of the incident radiation of the flame, particles, and hot gases. These mechanisms involve dependencies on the radiation spectrum due to the ash chemical composition and ash-sintering effects that enhance or hinder the overall heat transfer. In this paper, we discuss the complex mechanisms that describe the heat exchange in the presence of ash deposits and their impact on the prediction of the wall emissivity and ultimately the wall heat transfer in a real system.

Alkali metal emissions in an early-stage pulverized-coal flame: DNS analysis of reacting layers and chemistry tabulation

The intricate coupling between coal pyrolysis, gas phase combustion and the emissions of alkali metal, such as sodium, is studied in the early stage of a temporally evolving three-dimensional planar turbulent jet carrying pulverized-coal particles. Complex chemistry is used to account for both the combustion of volatile hydrocarbons and the sodium containing species. The response of the sodium chemistry is analyzed in the mixture fraction space, along with the topology of the reactions zones. Combustion is found to start preferentially in partially premixed flames, which then evolve toward diffusion-like reactive layers and reach chemical equilibrium. From the direct numerical simulation (DNS) database, the possibility of modeling the dynamics of sodium species using one-dimensional premixed flamelet generated manifolds (FGM) is investigated. A chemical lookup table is constructed for the combustion of the partially premixed volatiles and an additional three-dimensional simulation is performed to compare the tabulated sodium species against their reference counterparts with complex chemistry. Quantitative analysis of the performance of the developed chemistry tabulation confirms the validity of the approach. Perspectives for the modeling of sodium emissions in pulverized-coal furnaces and boilers are finally drawn.

Potentially toxic elements in fly ash dependently of applied technology of hard coal combustion.

Coal combustion is one of the most significant anthropogenic sources of thallium in the environment. This study presents the results of determination of thallium and some toxic elements (Pb, Cd, As, Ni, Zn, and Cu) concentration in fly ash produced in the coal combustion in conventional furnaces (pulverized coal furnace and grate furnace), in a fluidized-bed furnace with circulation bed, and in a home furnace. The high content of thallium was determined in ash produced in the grate furnace (on average 10.7mg/kg) and fluidized-bed furnace with circulation bed (6.6mg/kg). The average content of Tl in the fly ash of the pulverized coal furnace and in the home furnace is 1.7 and 2.4mg/kg, respectively. There was a strong relationship between the occurrence of Tl and As, Cd, Pb, Cu, and Zn in ash from grate and pulverized coal furnace.

Slagging and Fouling Characteristics of Zhundong High-Sodium Low-Rank Coal during Circulating Fluidized Bed Utilization

Zhundong coal (ZDc) with a large reserve is faced with severe slagging and fouling during combustion in pulverized coal furnaces because of its high Na content. Due to the low-temperature reaction characteristics of a circulating fluidized bed (CFB), the ash-related problems might be alleviated when ZDc is used as fuel in a CFB. In this study, the slagging and fouling characteristics of three types of ZDc were tested in a 0.4 t/d CFB test system. The influences of three aspects including reaction temperature, reaction atmosphere, and coal property were studied in order to realize the utilization of ZDc in the CFB. Experimental results indicate that slagging and fouling still occurred in the CFB. During gasification, ZDc displayed slagging behaviors in the riser, which was closely related to the used bed material of quartz sand. The slagging degree was increased with the rise of reaction temperature. In different reaction atmospheres, the migration and transformation behaviors of Na differed, which further...